APPARATUS 101.

150. Handles for Shocking Coils. Fig. 79. Ordinary sheet-tin makes good handles. Cut 2 pieces, each 6 x 4-1/2 in., and connect a stout copper wire to each. This may be done as suggested in Fig. 79, where the tin laps tightly over the bare end of the wire, or by punching 4 or 5 holes through the tin, and weaving the wire back and forth through the holes. Be sure that a tight and permanent connection is made. The wires joined to the handles should be about No. 20, and be 4 or 5 feet long. Roll the tin into a cylinder, so that the connection will be on the inside.



APPARATUS 102.

151. Handles for Shocking Coils. Very neat handles may be made from 4-in. lengths of brass tubing that is about 3/4 in. in diameter. The wires leading to the coil may be soldered to the handles.

APPARATUS 103.

152. Current Regulator for Induction Coils. Fig. 80. If your coil gives too much of a shock with one cell of App. 3 or 4, you can pull the carbon and zinc partly out of the solution to weaken the shock, or you can use a water regulator. T is an ordinary tin tomato can nearly filled with water, L is a lamp chimney. One wire, A, is fastened to T directly, or by a spring binding-post. The other wire, B, is fastened to a piece of copper, C, which may be raised or lowered inside of L. D is a piece of pasteboard with a small hole in its center.

153. Use. If this apparatus be put anywhere in the primary circuit, the amount of shock can be regulated by raising or lowering C. When C is raised, the current has to pass through a longer column of water than it does when C is near the bottom of L. When C touches T, the current passes easily. If it were not for the chimney, the current would pass to the sides of T.



CHAPTER XII.

CONTACT BREAKERS AND CURRENT INTERRUPTERS.

154. Contact Breakers; Current Interrupters. It is often necessary to make and break the electric current at frequent intervals. This can be done by an ordinary key (App. 118) by rapidly raising and lowering it. It is more convenient, however, to use some other form of apparatus. The current may be interrupted automatically; that is, it may be made to do the work itself (App. 100), or each make and break in it may be governed by the student.

APPARATUS 104.



155. Interrupter. Fig. 81. The body of this consists of a strip of wood, 6 or 7 in. long, 1-1/2 in. wide, and 7/8 in. thick. Cut a strip of tin 1 in. wide and long enough to bend down over the ends of the wood. Fasten the tin to the wood with small wire nails, driving the nails into the ends as well as into the top of the strip. Make a "center line" along the tin as a guide, and then drive 1-in. wire nails through the tin into the wood, so that they will make a row the length of the wood, and stand about 1/4 in. apart. On one end make a hole through the tin, and put in a screw-eye binding-post (App. 45). It is evident that if a wire from one pole of a battery be connected with the binding-post, it will also be electrically connected with the tin strip and nails. By touching the wire from the other battery-pole to the tin or to any nail, the circuit will be closed. If this last-mentioned wire be drawn along entirely above the tin, so that its end can bump along from one nail to another, you can see that the current will be closed every time a nail is touched, and be opened every time it jumps through the air. This apparatus can be connected with shocking coils, induction apparatus, etc., etc. Its use will be more clearly shown in connection with such apparatus.

APPARATUS 105.



156. Interrupter. Fig. 82. The nails in this apparatus are placed in a circle about 4 in. in diameter. They are electrically connected to each other by a bare copper wire, which is wound around each nail several times, and then led out to one of the binding-posts. In the center of the circle is a nail, or screw, which is connected by a wire to the other binding-post, care being taken not to allow the two wires to touch each other. Around the central screw is wound one end of a stout wire, the other end of which reaches out from the screw far enough to touch the nails. When this stout wire touches any nail, a current entering one binding-post can pass through nails, screw, etc., and out at the other binding-post. When the end of the stout wire is between two nails, the current cannot flow. By placing the finger against this stout wire and turning it around rapidly, the current can be interrupted as desired. The base should be about 5 x 6 x 7/8 in.

APPARATUS 106.

157. Interrupter. Wind the end of the wire from one pole of the battery around the handle of the file. Scrape the other wire along the rough file. As it jumps from one ridge to another the current will be rapidly interrupted.

APPARATUS 107.

158. Interrupter. Hold the end of the wire from one pole of a battery upon a saw-blade. Draw the other wire along over the teeth of the saw. As the wire jumps from one tooth to the next the current will be broken.

APPARATUS 108.

159. Automatic Interrupter. An ordinary electric bell, or buzzer, may be used as an interrupter. Every time the vibrating armature swings, the circuit is opened. The combination of a battery, induction coil, and electric bell makes a very good outfit for medical purposes. The automatic interrupter used on App. 100 should be studied.



CHAPTER XIII.

CURRENT DETECTORS AND GALVANOMETERS.

160. Current Detectors; Galvanometers. When a wire carrying a current of sufficient strength is properly brought near a magnetic needle, the latter will be deflected from its N and S line. The conducting wire has a magnetic field while the current passes through it, and this gives the wire the power to act upon a magnetic needle just as another magnet would.

The action of detectors, etc., depends upon this fact; and, strange to say, the magnetic field about the wire disappears the instant the current ceases to pass. The combination, thus, of a coil of wire and a magnetic needle, properly arranged, makes an instrument with which the presence of electricity can be detected. When the strength of a current is to be measured, or the strengths of two currents are to be compared, the apparatus is called a galvanometer. The method of making these pieces of apparatus will depend upon the strength of current to be tested or measured.

APPARATUS 109.

161. Current Detector. Figs. 38 and 40 show magnetic needles. These may be used to detect a current by holding the conducting wire near them and parallel to the needle. This form is not sensitive to weak currents. The delicacy of the apparatus is increased by allowing the wire to pass above and below the needle several times as in the next apparatus.

APPARATUS 110.

162. Current Detector. Fig. 83 consists, like all detectors, of a coil and a magnetic needle. The other parts are merely for convenience. Each turn of the coil helps to move the needle when the current passes.



163. The Coil is made by winding 10 feet of No. 30 insulated copper wire around the end of a broom-handle or other cylinder that is about 1 inch in diameter. This length of wire makes about 32 turns around such a cylinder. The exact length of wire for this makes no difference. After winding it, the coil should be slipped from the handle, being careful to hold it in such a way that it cannot uncoil and spring away from you. Tie the coil together with thread, in 3 or 4 places, to keep it in shape, and leave 5 or 6 in. of wire free at each end, so that connections can be made with other pieces of apparatus. After this is done press the coil into the shape shown, Fig. 83. This brings the wire near the needle and allows a longer needle to be used. The coil may be fastened to a pasteboard base. To do this, prick 4 holes in the base near the ends of the oval coil, and pass a strong thread through these with the aid of a sewing-needle. Tie the thread on the underside of the base at each end. If this is well done, the coil will be held firmly in an upright position. Paraffine may be used instead of the thread.

The ends of the wire should be made bare, and these may be sewed to the base to keep them in place.

164. The Needle may be supported upon a pin or needle-point. The piece of needle should be stuck through a cork which has a slot cut into its underside, so that it will straddle the lower part of the coil. The height of the needle-point should be fixed so that the horizontal ends of the magnetic needle will be near the axis of the coil, that is, along its central line.

165. To Use the Detector, turn its base around until the coil is in the N and S line—that is, until the magnetic needle is parallel to the length of the coil and wholly inside of it. Touch the ends of the coil with the two ends of the wire, which is supposed to carry a current. The needle will fly around until it is nearly perpendicular to its former position, if the current is strong enough.

APPARATUS 111.



166. Current Detector. Fig. 84. To make a more substantial detector than App. 110, the coil should be fastened to a wooden base. The coil may be made of 10 ft. No. 30 wire, as explained. (Sec. 163.) A hole should be made in the base with a small awl or with a hot wire, and into this should be set a pin, head down. The hole need not be larger than the pin-head, and when you find out how high the pin-point should be above the base, the pin may be fastened in place with a little paraffine, which should be pressed into the hole around the pin. The coil may be fastened in place with paraffine. The ends of the coil may be connected with binding-posts, described in App. 46, as shown, or with any other desired form.

The base should be 4 x 5 x 7/8 inches. The coil looks well when placed about 1 in. from the edge of the base. The binding-posts may be about 1 in. from the edges.

APPARATUS 112.

167. Current Detector. Fig. 85. This is more troublesome to make than App. 111, but perhaps it looks more scientific.

168. The Coil is wound around 2 ordinary spools which are glued to a vertical piece, which, in turn, is screwed to a base. You should not use iron nails or screws in the construction of electrical apparatus, when a magnetic needle is to be used in connection with it, as these would attract the needle. The spools may be pushed onto dowels which are fastened into the vertical piece. Small brass screws are good for the purpose also, if you haven't good glue or the dowels. This coil, etc., may be used in connection with an astatic needle. The coil may be wound with App. 93 or 94, if you make the attachment of App. 95, and screw the upright carrying the spools to the attachment.



The binding-posts, shown in Fig. 85, are not to be advised. It will be better to use those of App. 45. The magnetic needle is supported by a sewing-needle stuck through a cork. This may be fastened to the base with paraffine.

169. It is often troublesome to turn the apparatus around until the needle becomes parallel to the length of the coil. To avoid this, a small bar magnet, shown in the Fig. 85, may be laid on top of the coil. A magnetized sewing-needle will do, and this will keep the magnetic needle quiet and parallel to it when the current is not passing through the coil. Of course, it takes a little more current to move the magnetic needle when the bar magnet is in place, than it does without the magnet.

170. By allowing the current to enter the right-hand binding-post, as you look at it from the front (Fig. 85), it will go around the coil in the direction of the hands of a clock, that is, from left to right on top. This, of course, is not necessary to merely detect the presence of a current. In order, however, to determine the direction of currents by means of a magnetic needle, study the effect with a single turn of wire at first. (See text-book.)

171. Dimensions. The base is 5 x 4 x 5/8 in. The upright piece is 5 x 3-1/2 x 5/8 in. The spools are 2-1/2 in. apart center to center.

APPARATUS 113.



172. Astatic Current Detector. Fig. 86. The ordinary magnetic needle points to the north quite strongly. It is evident, then, that this pointing-power must be overcome by the magnetic field around the coil of wire, before the needle can be forced from the N and S line. Very weak currents will not visibly move the magnetic needle in the detectors so far described. You should remember that no action will take place unless the magnetic field around the magnetic needle is acted upon by that around the coil. In order to make an instrument that will be very sensitive, we must have strong fields about the needle and coil, and we must, at the same time, decrease the pointing-power of the needle. We can increase the strength of the field about the needle, and at the same time decrease its pointing-power by using an astatic needle. (See App. 69.) The arrangement shown in Fig. 86 is a very simple one, and it is quite sensitive.

173. Details of Construction. The base is 4 x 5 x 7/8 in. The coil is made from 10 ft. of No. 30 insulated copper wire. (See Sec. 163 for details about coil making.) The binding-posts are like App. 41. The Astatic Needle is described for App. 69. The needles may be broken off, if too long for the coil. They are supported by a fine thread hung from a screw-eye, which may be turned to adjust the position of the needles. This is not necessary, as the thread may be hung from a plain wire arm that reaches out from the upright rod. This rod is a 6-in. piece of dowel, 1/4 or 5/16 in. in diameter. It stands in an ordinary spool which should be glued to the base. Do not nail it to the base. The wire arm may be of iron, as it is some distance above the needle; but it is better to use a stiff brass or copper one. In the figure one end of the wire is twisted around the screw-eye, making a nut for the screw-eye to turn in.

Hang the astatic needle so that the wire between the two parts will not quite touch the coil. The needles should be parallel to the coil before testing for currents. They will fly around very decidedly with even fairly weak currents.

APPARATUS 114.



174. Astatic Current Detector. Fig. 87. For a description of the wood-work, coil, etc., see App. 112; for the astatic needle see App. 69; for the method of supporting the needle see App. 113, Fig. 86. The top part of the coil is spread apart a little to allow the lower needle to be dropped through the opening thus made, and to allow the wire joining the two needles to be free to turn. The needles may be broken off a little, if necessary, or an opening may be cut into the vertical part of the frame, so that they can swing more freely. This detector will indicate quite feeble currents.

APPARATUS 115.

175. Astatic Detector. Fig. 88. As previously Stated, the sensitiveness of a detector can be made greater by increasing the strength of the coil-field for a given current. This may be done by increasing the number of turns of wire in the coil. The most convenient way will be to use two coils, one on each side of the astatic needle.

176. The Support, or framework, is a lamp chimney. By this the astatic needle is suspended and protected from air currents. The chimney should be at least 3 in. in diameter at the bottom, about 10 in. high, with a plain round top. Upon the top of the chimney is placed the cover of a wooden pill-box, 2 in. in diameter.

177. The Coils should be made separately, for convenience. Each should be of 10 ft. No. 30 wire. (See details Sec. 163.) Cut out a round piece of stiff pasteboard, just large enough to go inside of the bottom of the chimney. Fasten the coils to this by sewing (Sec. 163), or with paraffine, so that they shall be symmetrically located and 3/8 in. apart. The pasteboard circle may be fastened to the base with small brass screws. Do not use any iron nails or tacks. In this, all four ends of wire are brought out under the edge of the chimney (Fig. 88). Cut little grooves in the base for the wire to sink into, so that the chimney will rest firmly upon the base all around. The ends of the wires are fastened to three binding-posts.



178. Joining the Coils. The end of one coil must be joined to the beginning of the other properly, or the action of one will destroy that of the other. Fig. 89 shows the two coils, A and B. If the current enters at the binding-post, X, it will pass through the turns of coil A, in the direction of clock-hands, then out to Y, where B begins, around B in the same way, and then to Z. Y may be simply a screw-eye binding-post (App. 41). By this arrangement one or both coils can be used at a time. If the current is very weak, use both coils; that is, connect the ends of wires to be tested with the two outside binding-posts. If they are joined to the middle and one outside post, one coil only will be in the circuit.

179. The Base should be about 7 x 5 x 7/8 in. Fasten three bent brass or copper strips to the base with brass screws to hold the chimney steady. By bending them in more or less you can make a snug fit around the chimney.

180. Adjusting the Needle. In the center of the box-cover is a small hole. The thread from the needle passes through this. The upper end of the thread is wound around a screw-eye, which is screwed into the cover near one edge. By turning the cover around, the needle can be made to hang parallel to the coils, and by turning the screw-eye, the needle can be raised or lowered. A small hole should be made in the cover before putting in the screw-eye, or you will be liable to split the wood.



181. Use. This apparatus will indicate very slight currents; in fact, as feeble ones as the student will have occasion to experiment with, such as induced currents, currents of thermo-electricity, and currents produced by exceedingly weak batteries. (See text-book.)

APPARATUS 116.

182. Tangent Galvanometer. Fig. 90. For the uses of this form of galvanometer see text-book. Do not use any iron in making this apparatus. The base is 5 x 4 x 7/8 in. At its front end are three binding-posts. The pasteboard band, G, is 1-1/4 in. wide and 6 in. in diameter. Cut the pasteboard 21 in. long and 1-1/4 in. wide, then bend it into the form of a circle. There will be a lap of about 3 in., and you can make it solid by sewing the two ends together at the lap.



183. The Coils maybe made of No. 24 insulated copper wire, which should be wound on before fastening G to the base. There are two separate coils, one having five turns and the other ten turns. Leaving a 6-in. length, A, for connections, wind five turns of wire on to G, putting them on clockwise; that is, pass them over the top of G from left to right. Tie thread around G and the wire to hold them together after you have five turns on, and cut a 6-in. end, B. Now begin with C, and wind on ten turns, bringing the end of them out at D. Punch holes, F, through G on each side of the coils, run twine, T, through them, and tie T on the outside of G. Do this in three or four places, to firmly hold the coils.

184. Fastening Coils to Base. The band and coils will not rest squarely upon the base, so cut two pieces of wood, E, about 2 x 1/4 x 1/4 in., to be put under G, one being on each side of the coil. Make holes through the base, pass strong cord, H, through them, and over the inside of G, then tie under the base. This should tightly squeeze E, and hold G upright and firm.

185. The Connections. A and B are the ends of the five-turn coil; C and D are the ends of the ten-turn coil. If the battery-wires are connected with X and Y, the current will pass through five turns of wire; if connected with Y and Z, it will pass through ten turns; if with X and Z, the current will pass through the entire fifteen turns. In this way the strength of the magnetic field about the coil can be regulated, and its effect upon the magnetic needle, M, changed.

186. To Support the Needle, glue or sew two strips, I, to G. They must be in such a position that the poles of M will be as nearly as possible in a horizontal line drawn through the center of the circle, G. After you have made M (App. 66), and have found where the pieces, I, should be, fasten them to G, and then to I glue a pasteboard strip, J, about 1-1/4 in. wide. Run a pin, P, up through the center of J to support M.

187. The Magnetic Needle, M, should not be over 1 in. long for this kind of an instrument. (See App. 66 for full directions for making it.) On the top of M should be fastened a light paper pointer or index, L. The short end should be made large, so that the long slim end will not over-turn M; that is, the pointer should balance itself. It may be fastened to M with paraffine or a drop of sealing-wax. If carefully balanced, the pointer can be made quite long.

188. The Graduated Circle, K, is described. (Index.) With this you can tell through how many degrees the needle is deflected, when the current passes. The strength of different currents can be compared, and many interesting experiments performed with the tangent galvanometer. For clearness, the circle, K, is shown small. In order to have the divisions on it far enough apart, K should be about 4 in. in diameter. The zero points should be at the front and back of the instrument, when a pointer is used on the needle.

189. How to Use It. For full explanations, and for the study of experimental cells, etc., by means of the tangent galvanometer, see text-book. It will be impossible for you to get M exactly in the center of G; you cannot get the pointer exactly at right angles with M; hence, if you pass a certain current through the coils, and the pointer reads 20 degrees, you will find, if you reverse the current, making it go through the coil in an opposite direction, that the pointer may read 24 degrees on the opposite side of the zero. To get the true reading, then, take the average of the two, which in the case mentioned would be 22 degrees. (See current reversers.)

APPARATUS 117.

190. Tangent Galvanometer. Fig. 91. The base consists of 2 parts, A and B. It is not necessary to use two pieces if you have wood that is at least 7/8 in. thick. This is given as a suggestion in case you have nothing but thin boards. By screwing B to A the base is made thick enough to take the screws for binding-posts. The base proper, A, is 8-1/2 x 5 x 1/2 in. If you make this of 7/8 in. stuff, you will not need B.

The Back, C, is 10 x 8-1/2 x 1/2 in. It is screwed to the base. Do not use nails, as these affect the magnetic needle. Find the center of C, and with this as a center, draw two circles, (that is, the circumferences of two circles,) one 5 in. in diameter to show where to cut out a hole, H, and the other 7 in. in diameter to serve as a guide for fastening on the spools, F.



The Spools, F, are glued to C. If you have brass screws, these may be used instead of the spools; they should be left sticking out from C about 1 in. Around the spools or screws, fasten a pasteboard band, G, on which to wind the wire. G may be about 1 in. wide; it should be kept in the circular form by sewing the ends together where they lap. (Read directions in App. 116.)

191. The Coils on this model are 4 in number. (See App. 116 for the method of winding.) The first coil is made of coarse wire, No. 18, its ends being joined to the binding-posts, V and W. The second coil has 5 turns of No. 24 insulated copper wire, its ends being joined to W and X. The third coil has 10 turns of the same size wire, No. 24, and is joined to X and Y. The fourth coil has 20 turns of the same joined to Y and Z. If you want to use the galvanometer for quite weak currents, it would be well to make a fifth coil of 20 turns of No. 30 wire, and join it with Z and a new binding-post. The ends of the coils are run through small screw-eyes before passing to X, Y, etc. This is not necessary, it merely keeps them in place.

The Binding-Posts are like App. 43. Any other desired style may be used, those of App. 46 being preferred.

The Hole, H, is 5 in. in diameter. It should be cut out about 1/2 in. below the center of the circles to allow for D, and for the pin-point which supports the magnetic needle, the poles of which should be in the line passing through the center of the coils. The method of cutting the hole, H, through C, will depend upon the tools at your service.

D is the front edge of an adjustable table, like that explained. (Index.) It is 4-1/4 in. wide. It supports the magnetic needle which is inside of E.

E is the outside of a glass-covered compass. (See App. 67 for details.) The needle should not be over 1 in. long.



CHAPTER XIV.

TELEGRAPH KEYS AND SOUNDERS.

APPARATUS 118.



192. Telegraph Keys. Fig. 92. Telegraph keys are merely pieces of apparatus by which the circuit can be conveniently and rapidly opened or closed at the will of the operator. An ordinary push-button may be used to turn off and on the current, but it is not so convenient as a "key." Fig. 92 shows a side view of a simple key. C is a metal strip about 3/4 in. wide and 4 or 5 in. long. At the left end it is fastened to the base with a screw, A. Another screw, X, serves as one binding-post. Y is another screw binding-post. W is a short wire, used to regulate the amount of spring to the key. This is done by moving W to the right or left. If the current enters at X, it will pass along C and out at Y, when C is pressed down. By moving C up and down according to a previously arranged set of signals, messages can be sent by means of the electric current. (See telegraph alphabet.) This apparatus is not a good one where the line is to be run with a "closed circuit battery," or where it is to be used very often. It will do, however, for places where a push-button would be too tiresome to use. The right end of C is curved. This curve serves as a handle. D and E are wires leading from X and Y.

APPARATUS 119.

193. Telegraph Key. Fig. 93. The base is 5 x 4 x 7/8 in. The key, C, is made of two thicknesses of tin. It is made into a strip 5-1/2 x 3/4 in., then the front end is bent up for a handle, as suggested in Fig. 92, the front end being above the base so that it will not touch the strap, D, unless it is pressed down. C is fastened to the base by a screw, H, which also binds one end of the copper wire, C W. About 3/4 in. from H is placed X, which is a screw-eye binding-post. Under C is the wire, W, which is used to regulate the amount of spring in C, by moving it forward or backward. S I shows the position of a screw-eye, or of an ordinary screw put into the base through C. The hole in C should be made so that C can move up and down easily around the screw. This is used to make a click when the key is allowed to spring up. The downward click is made when C strikes D at each depression.



The Strap, D, is made of tin. It is 4 x 1/2 in. before bending up the right end a little. It is fastened to the base by the screw, F, and by the other binding-post, Y. Its right end is raised enough to allow the arm, E, to pass under it, but it must press down well upon E when E is forced toward F.

The Swinging Arm or Switch, E, is also made of tin, and measures, finished, 4-1/2 x 1/2 in. Its front end should be bent up a little for convenience in handling it. (See Fig. 92.) E is pivoted at G by a screw, which also binds the wire, C W. Fig. 24 shows another way to make the pivot and connection.

194. Operation. See Fig. 99 for the details of the connections of a home-made telegraph line. When you are using the line and telegraphing to your friend, the switch, E, of your instrument must be open, as in Fig. 93, and the corresponding switch on his instrument must be closed; that is, the circuit must be opened and closed at but one place at a time. As soon as you have finished, your switch must be closed. He will open his and proceed. When you have both finished, both switches must be closed. If your friend left his switch open, you could not call him over the line, as no current could pass into his sounder.

195. Batteries. As the circuit has to be left closed for hours and perhaps days at a time, so that either operator can call the other, a closed-circuit battery is necessary. (See App. 9.) A dry cell, Leclanche, or other open-circuit cell would not be at all suitable for a telegraph line, as it would soon polarize. Large Daniel cells, which are 2-fluid cells like App. 7, or gravity cells (App. 9) are the best for your line.

APPARATUS 120.

196. Telegraph Sounder. Fig. 94. The wood-work consists of 2 parts; the base, B, is 6 x 4 x 3/4 in., and the back, A, is 6 x 5 x 1/2 in. A is nailed or screwed to B.

The Magnet, M, is fully described in App. 85. M is held firmly to A by cord or wire, which should pass around it near the poles and at the curved part. The wire should pass through small holes in A, and be tied at the back. Wire nails driven into A at the sides of M will keep it from moving about. The wires from the magnet coils are led to two spring binding-posts, X and Y.



197. The Armature, C, is made of a narrow piece of thin iron, about 5-1/2 x 1/4 x 1/8 in. It may be made by bending up 3 or 4 thicknesses of tin into that shape. This is the part which will be attracted by M, when the current passes, and which will make the clicks by which the message can be read. (See telegraph alphabet.) There are many ways by which C can be held near M. The figure shows how it can be done entirely with 1-in. wire nails. At the right end of C two nails are driven into A above and below C. They are just far enough apart to allow the left end of C to be raised and lowered without binding; in other words, these nails make a pivot for C to swing upon, and they help to support it at the same time. The left end of C must not quite touch the poles of M when the current passes, because the residual magnetism would keep C from dropping back into place. To adjust the armature, pass the current through M, hold C so that it will not quite touch the poles, then drive in the upper nail, 2. Put another nail, 1, below C, so that M will not have to lift C more than 1/8 or 3/16 in. Try the nails in different positions until C quickly rises and falls when the circuit is closed and opened. A nail, 3, driven in front of C, will keep its right end in place. No springs are needed, as gravity acts upon C instantly, bringing it to the lowest position as soon as the current ceases to flow.

198. The Battery will depend upon how much you want to use the sounder. If just to show the principle of it, almost any cell of medium strength will do, like that of App. 3, 4 or 5. A dry battery will do, but if you use the sounder much, an open-circuit battery will soon use itself up. Where much work is needed of the battery use App. 9.



The Key like App. 119 is best. Push-buttons are handy where used only for experiments, and not for the actual sending of messages.

APPARATUS 121.

199. Telegraph Sounder. Fig. 95. This makes a simple and efficient sounder for short lines. The base, B, is 7 x 4-1/2 x 7/8 in. The back, A, is 7 x 4-1/2 x 1/2 in.; it is nailed to B. The piece D is 4 x 3/4 x 3/4 in.; it is nailed to A. C is a wooden piece 1-1/2 x 3/4 x 3/4 in.; it is nailed to A, and in its top is a screw, E, which is used as a regulating-screw to keep the armature, L, from touching the poles.

200. The Armature, L, is explained as App. 77. The two thicknesses of tin at F must not be too thick, or it will take too much battery power to work the sounder. If you find that it is too stiff to bend down, when the current is on, try the arrangement of App. 122, which is easier to make and regulate. The whole point depends upon the tin you have. The end of L must tap against E. A hole is punched in the part F, and a screw, G, holds it to D. L should rest about 1/8 in. above the poles and gently press against a screw or nail, V.

201. The Magnets are like App. 89. They are made as in App. 88, and held down like App. 90. These should be placed very near the back, A, so that the armature will be over them. If your yoke is not too wide the coils may rest against A. Y and Z are binding-posts like App. 46.

202. Connections. Join the coils as explained in Sec. 125 and see Sec. 115. Instead of a third or middle binding-post, as in Fig. 66, hold the two inside ends between a screw-head and a copper bur. The method of joining the wires for a line with two outfits, is shown in App. 124. If you have but one key, sounder, and battery, simply join the line wire to the return wire there shown. A gravity cell is best. (See App. 9.)

203. Hints About Adjusting. If you have the right spring to the part F, of the armature, you will have no trouble. It must not be so weak that it allows L to strike upon the poles, as the residual magnetism (Text-book) will hold L down after the current has ceased to pass. No springs are necessary, if your tin is right. Do not have L too far away from the poles. The distance is regulated by the position of V. If you have trouble in getting it to work see App. 122. The poles must be opposite in nature.

APPARATUS 122.



204. Telegraph Sounder. Fig. 96. The magnets, connections, etc., are like those of App. 121, no binding-posts, etc., being here shown. The armature is straight, however, the part F resting upon D. A hole is made in the end of F, and through this is a screw or nail, S. The hole must be large enough to allow S to pass through easily. This acts as a bearing or pivot. L is kept up against V by the rubber-band, J, one end of which passes around the end of L; to the other end of J is a thread, which is tied around a screw-eye, K. By turning the screw-eye, the band may be made to pull more or less upon L. In this way the apparatus may be regulated according to your battery. The general dimensions and explanations are given in App. 121. D is made of such a height that it will bring L about 1/8 or 3/16 in. above the poles.

APPARATUS 123.



205. Telegraph Sounder. Figs. 97 and 98. This apparatus looks a little more like a regular sounder than App. 121 and 122, but it is much harder to make and adjust. In this the lower nuts of the bolts are not sunk into the base, and the magnets are made of 2-in. bolts. If you change this and fasten them like App. 89 and 90, it will simply change the dimensions of the small parts. The sizes given are for this particular instrument.

Fig. 97 shows a perspective view, and Fig. 98 is a plan or top-view of it, with dimensions.



206. The Base, B, is 6 x 4 x 7/8 in. The magnet, M, is explained in App. 89. Its wires are attached to the binding-posts like App. 46. The armature, A, is 2-1/2 x 3/4 x 1/8 in., and made as described in App. 71. The piece, D, is 2-1/2 x 1-3/8 x 1/2 in., and is screwed to B from below, after the two uprights, C, are nailed to it. The uprights, C, are 2-3/4 x 7/8 x 1/2 in. They are nailed to D. The nail, N, runs through both uprights, and acts as the bearing for F to rock up and down upon. The hole for N is 2 in. above B. It must not be too loose in the holes, or F will rock sidewise, and allow A to touch one of the magnets. The upright, E, is 2-3/4 x 3/4 x 3/4 in., and is screwed or nailed to B from below. A screw, G, is put into the side of E near the top. This screw has the underside of the head filed flat, and against this the screw, L, taps when the armature is attracted. The arm, F, which carries the armature, A, is 4-1/2 x 1/2 x 1/2 in., and is pivoted by means of N, which passes through it and the uprights C. F must swing up and down freely. The hole for N, in this model, is 1-3/4 in. from the armature end.

207. The armature is fastened to F by a screw, S. A copper bur is put under the head of S to aid in keeping A from rocking sidewise. Through F, and about half way between C and L, is put a screw, I, the lower end of which taps against the head of a screw, H, which is put into D. By unscrewing H a little, F will be raised, and A will be brought nearer the poles of M. The rubber-band, J, is placed over the head of I, and has tied to it a thread, O, which in turn is tied to a screw-eye, K. K screws into the end of B, and by turning it one way or the other, the tension, or pull, on J may be increased or diminished. There must be enough spring in J to pull A up after the current ceases; it must not pull so much that the magnet cannot draw A down hard enough to make a good click between L and G.

The Magnet, M, is explained in App. 89, and the construction of one bolt magnet is given in detail in App. 88. In this particular sounder the bolts are 2 in. long under the heads, thus bringing the tops of the bolt-heads about 2-1/4 in. above B. M is held to the base by a band of tin, T. The yoke may be screwed to B, as suggested in App. 90. This is the better plan.

208. Adjustment. You will find, although you make all of the parts with the dimensions given, that you will have to try, and change, and adjust before everything will work perfectly. A must not be allowed to touch the poles of M when it is pulled down, on account of the residual magnetism, which would keep it pulled down. Adjust this with F. The armature must not be pulled too far up from the poles of M by the tension in J; adjust this with I and H. If your battery is weak, the pull of J must be small, just enough to raise A.

The Battery. It is supposed, if you make an instrument like this, that you expect to use it for a line. In that case make a regular gravity battery like the cell of App. 9. See Fig. 99 for line connections, and Fig. 98 for plan view of this sounder.

APPARATUS 124.

209. Telegraph Line; Connections. Fig. 99 shows the complete connections for our telegraph line, with two complete outfits. The capital letters are used on the right side, R, and small letters are on the left side, L. The batteries, B, b, are like App. 9. The keys, K, k, are like App. 119. The sounders, S, s, are like App. 121 or 122.



210. The two stations, R and L, may be near each other, or in different houses. The return wire, R W, passes from the copper of b to the zinc of B. This is important. If the cells are not joined properly, they will not work. It is better to have the cells together, on a short line, joined in series. The line wire, L W, and the return wire, R W, may be made of insulated copper wire for short lines in the house. Ordinary annunciator wire, No. 20, is good and cheap. The kind that is double cotton wrapped, waxed, and paraffined, has about 235 ft. to the pound. You should get at least 5 ft. for 1 cent. If your line stretches from one house to another you will find it better to use iron wire. Galvanized iron or steel wire No. 14 is good. This size weighs about 100 lbs. to the mile. The return and line wires must not touch each other at any point; they must not touch any pipe or other piece of metal that will short circuit your batteries. It is best to use porcelain or glass insulators to support your wires if the line is long; but for short lines, where you use a return wire, you may support the wires upon poles or trees by means of loops made of strong cord or wire.

211. Operation. Suppose R (right) and L (left) have a line. By studying Fig. 99 you will see that R's switch, E, is open while e is closed. The whole system, then, has but one place where the circuit is open. As soon as R presses his key, K, the circuit is closed, the current from both cells rushes around through K, S, L W, s, k, b, R W, and B. This magnetizes the bolts of both S and s, and their armatures come down with a click upon the regulating-screws, where they remain as long as the current passes. As soon as R raises his key the armatures rise, making the up-click. R can, in this way, regulate the time between the two clicks. If he presses K down and lets it up quickly, the two clicks that his friend L hears from s are close together; this makes what is called a dot. If R holds K down longer, it makes a longer time between the clicks for L to hear, and this makes a dash. R, of course, hears his own sounder, which is making the dots and dashes also.

As soon as R has finished, he closes his switch, E. L then opens his switch and proceeds to answer. Both E and e should be left closed when you are through talking.

(Read Sec. 194, 195, and study what is said in App. 9 about the gravity cell to be used on such a line.)

212. Telegraph Alphabet. The letters are represented by combinations of dots, dashes and spaces. A dot is made by pressing the key down, and raising it at once; that is, the key is raised as soon as it strikes. This makes the letter E. The dash is made by pressing down the key, and allowing the current to pass about as long as it takes to make 3 dots; this makes the letter T. A long dash for L should take about as long as for 5 dots. Spaces occur in a letter and between words. To make a dash you hesitate while the lever of the key is down, to make a space, you hesitate while the key is up. H is made with 4 dots without hesitation or space. By putting a space between the dots the letter &, Y or Z is made according to the position of the space. Notice that letters containing dashes do not contain spaces. A space is really the opposite of a dash. The letters C, E, H, I, O, P, R, S, Y, Z, and & are made entirely of dots or of dots and spaces.

You should notice that several letters are the reverse of others; A is the reverse of N, B of V, D of U, C of R, Q of X, and Z of &. The student should study some book upon telegraphy, if he desires to become expert. Punctuation marks are left out of the alphabet here given, as boys will find very little use for them.

A _ _ B _ _ _ _ C _ _ _ D _ _ _ E _ F _ _ _ G _ _ _ H _ _ _ _ I _ _ J _ _ _ _ K _ _ _ L _ M _ _ N _ _ O _ _ P _ _ _ _ _ Q _ _ _ _ R _ _ _ S _ _ _ T _ U _ _ _ V _ _ _ _ W _ _ _ X _ _ _ _ Y _ _ _ _ Z _ _ _ _ & _ _ _ _

1 2 3 4 5 6 7 8 9 0



CHAPTER XV.

ELECTRIC BELLS AND BUZZERS.

APPARATUS 125.

213. Electric Buzzer. Fig. 100. A buzzer is, in construction, very similar to an electric bell; in fact, you will have a buzzer by removing the bell from any ordinary electric bell. They are used in places where the loud sound of a bell would be objectionable. As the buzzer is easier to make than a bell, we shall discuss it first.

214. The arrangement of the parts, (Fig. 100), is very much like that of the sounder of App. 121, Fig. 95. The armature is, in this case, a vibrating one and acts on the same principle as the automatic interrupter on App. 100, which you should study. (See Sec. 148.) The general dimensions may be taken from App. 121. The base, B, in this case is about 1 in. wide. D also is made 1 in. wide. H is 1 x 1 x 1/2 in., and is nailed to A. Through its center is a hole for the regulating screw-eye, I. The end of I presses against F. The exact position of H will have to be determined after the magnets are in place. The armature, L, should be about 1/8 or 3/16 in. above the poles. They are not allowed to strike the poles, as a screw, E, regulates that. (See Sec. 203). Y and Z are two binding-posts, like App. 46. To these are connected the battery wires. The strip of tin or copper, which forms Y, is cut like a letter T there being three holes in it, one near the end of each arm. The screw-eye, 2, and the screw, 3, are put through the horizontal part of the T, and the regulating-screw, I, passes through the hole in the vertical part which springs up against I, thus forming an electrical connection between Y and I. The magnets are made and fastened as in App. 89.

215. Connections. The inside ends of the magnet coils, (Sec. 123), are fastened between a screw-head and a copper bur, S. One outside end goes to Z, and the other under the screw, G, which holds F to D.



216. Adjustment. The part, F, and the screw, E, must be just high enough to keep L from striking the poles of M. If F is too weak, it will bend down to M. If F is too strong, it will take too much battery power to run it. In case there is not strength enough in F to quickly raise L when the current ceases to pass, arrange a screw-eye and rubber band as shown in Fig. 96. I should be slowly turned one way or the other, until it touches F just right to allow L to vibrate back and forth rapidly.

217. Operation. We shall suppose that you have all parts adjusted and the battery wires joined to Y and Z. If the current enters at Z, it will fly around through the coils, through G, F, up I, through the T-shaped tin and out at Y. The current was in L, but it could not get out at any other place than at Y. As soon as the bolts were magnetized, L was forcibly drawn down, pulling F away from I, thus opening the circuit. As the bolts were no longer magnets, F sprang right back to I, the current passed long enough to re-magnetize the bolts. This operation was rapidly repeated.

218. Use. If you wish to use the buzzer simply to call some one occasionally, a dry battery or Leclanche cell is best. This apparatus is good to work a gravity cell when it needs regulating.

APPARATUS 126.



219. Electric Bell. Fig. 101. Before making this bell, carefully read the directions and explanations given for the electric buzzer, App. 125. The parts are very much alike in the two instruments, and most of the lettering of them has been made the same in the illustrations. If you look at Fig. 101 from the side, with the letters M and Q at the bottom, you will see that this bell is merely a modified form of App. 125.

The Base is 7 x 5 x 1/2 in. To the upper end of this is nailed the cross piece, D. To D are fastened the binding-posts.

The Parts, F, G, H, I, J, K, L, M, N, P, Q, are the same as explained in App. 121 and 125.

The Magnet is fastened to the base by a tin strip, C, which is screwed down at both ends. By nailing a strip, like D, along the left side of the base, the magnet may be fastened to this. This strip would take the place of the base of App. 125.

The piece, F, of two thicknesses of tin, is made longer than it was in App. 125; in fact, it projects through L and forms the part N. To the lower end of N is fastened a large bullet. Hold the cutting-edge of a strong knife-blade upon the bullet, and with a few taps of a hammer drive the blade into it to make a gash.

Put the end of N into the cut, then hammer the bullet so that N will be pinched. If you have no bullet, cut a long strip of tin, about 3/8 in. wide, and wind this about the end of N to serve as a ball.

The Bell, E, may be taken from an old alarm-clock. This is not screwed directly to the base, as it would not ring well. After you have the ball, O, properly fixed, hold E, so that O will strike it near its rim; then cut a piece of wood about 5/8 x 5/8, and long enough to put under E, to raise its rim to the right place. This piece must be screwed to the base from the underside, and on to its top is placed the screw which passes through the bell. In other words, E is mounted upon a rod which is fastened to the base.

The Adjustments are made as in App. 125. By bending N a little, O can be made to tap E properly.

The Battery for a bell that is to be used much should be an open circuit one, such as the Leclanche, or the ordinary dry batteries. It is cheaper to buy a dry battery than it is to make one suitable for bells. A and B show wires that lead to the bell from the battery. One of the wires should be passed through a push-button.

APPARATUS 127.

220. Electric Bell. By arranging the buzzer of App. 125 with a bell, you can use the same for an electric bell. The part, F, should be made long enough to extend entirely through L, and project beyond L for about 2 in. To the end of this is fastened a large bullet, or a band of tin. (See App. 126.)



APPARATUS 128.

221. Combination Buzzer and Telegraph Sounder. Fig. 102. This apparatus is good for experimental purposes, where you do not wish to go to the trouble to make two separate pieces. For the dimensions and explanations see App. 121 and 125. There is but a slight change in App. 125 to make this.

222. Connections. The inside ends (Sec. 123) of the magnet wires are fastened together at S. The outside ends are joined to the two binding-posts, Y and Z, made like App. 46. A wire, P, joins Y with the screw in T, which is a piece of stiff tin or copper, which presses down upon the top of I. In this way a connection may always be had between I and T. A wire, R, joins F electrically with X; it is held under the head of the screw, G. (See App. 125 about adjustments.)

223. Operation. When you wish to use the apparatus as a buzzer, join your battery wires to X and Z. If the current enters Z, it will pass through the magnet coils out to Y, through P, T, I, F, and R to X. If you use it as a telegraph sounder, join the battery wires to Y and Z. The current will then pass simply through the coils; it will not bother to go into P, F, etc., as it has no place it can escape. If used simply for experimental purposes almost any cell of sufficient strength will do. If for telegraph, use App. 9; if for buzzer, use an open circuit cell, as, for example, a dry cell.



CHAPTER XVI.

COMMUTATORS AND CURRENT REVERSERS.

224. Commutators and Current Reversers are useful in some experiments, as, for example, those with tangent galvanometers (App. 116, 117), in which readings are made with the current passing around the coil in one direction, and again made at once with the current reversed. The use of commutators on motors and dynamos should be understood. The reversers herein shown are, of course, not at all like those used on motors. Current reversers are used in connection with the needle-telegraph and many other instruments.



APPARATUS 129.

225. Current Reverser. Fig. 103. The base is 5 x 4 x 7/8 in. To this are fastened four metal straps, A, B, C, and D. These may be made of brass, aluminum, or even of tin. If made of tin, use one thickness of metal for C and D, and two thicknesses for A and B. Each strap has two 1/8 in. holes punched in it, their positions being shown by the screw-heads and screw-eye binding-posts.

Construction. C is 3-3/4 x 1/2 in. Fasten this to the base first. At the left end is a small screw, while the right end is held down by the binding-post, W. The keys, A and B, should have quite a little spring to them. These are cut 5 x 3/4 in. The front end of each is bent over a little (see the key App. 118, Fig. 92) so that they may be more easily grasped. The length after bending will be less than 5 in. The front ends should be raised from the base (Fig. 92) so that they will not touch C, unless pressed down. The 1/8 in. holes in the end of A are about 3/4 in. apart, one being used for a screw to hold it to the base, and the other for the binding-post, Y. The strap, D, is 3-3/4 x 1/2 in. It is fastened at one end by a screw, and at the other end by X. D is bent about 3/4 in. from each end, so that its middle part stands above the base about 1/4 in. The straps, A and B, press up against D, unless they are held down with the hand.

226. Connections. W and X are joined to the poles of the battery to be used. Y and Z are joined to the apparatus in which the current must be passed in one direction, and then in the opposite direction. A tangent galvanometer, or a needle-telegraph instrument, for example, may be connected with Y and Z.

227. Operation. Suppose that the battery current enters at W. As long as both keys are raised, the current can go no farther. Now, imagine that we press A down solidly upon C, the current will pass along A, which does not now touch D, out through Y into the galvanometer, back to Z, into D, and to the battery again; that is, the current will enter the galvanometer from Y. Now, suppose that we let A spring up against D again, and press B down, the current still coming into W from the battery; the current will pass along B, out through Z, into the galvanometer, back to Y, through D, and back to the battery. It is evident, then, that the current can be made to pass out of Y or Z to the galvanometer at will by pressing down A or B.

APPARATUS 130.



228. Current Reverser. Fig. 104. The wooden base is 7 x 5 x 7/8 in. To this are fastened two brass or tin straps, C and D, 5 x 1/2 in. They are fastened at the front ends by screws, S, while the binding-posts, Y and Z, hold the other ends solid. X and W are two screw-eye binding-posts (App. 45). The small square piece of wood, T, is 3 x 3 x 1/2 in. Through the corners of T, and in positions so that they will be directly over C and D, are put four screw binding-posts, 1, 2, 3, 4 (App. 41). The screws, however, pass entirely through T, and stick out about 1/4 in. on the underside of it. The wire, A, connects W, 1 and 4, while the wire, B, connects X, 2 and 3. A and B must not touch each other where they cross on the top of T. N is a wire nail that serves as a handle. If we were to place T, holding the four corner screws, upon the straps, C and D, it is evident that all the screws would touch the straps, if they were properly adjusted. We must fix things so that two only can touch the straps at a time. Put a screw, Q, through the center of T, from the bottom, so that it will stick out of the bottom more than the screws, 1, 2, etc. The screws, 2 and 4, will be lifted from C and D when the handle, N, is pressed down. By raising N, the top, T, can be made to rock up and down upon Q as a pivot. By lifting N far enough, 2 and 4 will be pressed against C and D, while 1 and 3 will be raised. A spring, R, is shown joined to T and to the base. This will hold the screws, 2 and 4, down upon C and D, unless N is pressed down.

229. Operation. We shall first suppose that the spring, R, is holding 2 and 4 in contact with C and D; 1 and 3 will, of course, be held up in the air. Imagine that we have a galvanometer connected with Y and Z. If the battery current enters at W, it will pass along A to 4, before it can find a chance to escape. It will pass through 4 into D, and into the galvanometer by way of Z, then back by way of Y, up 2, and out to the battery from X. If we now press the handle, N, down, the current will pass from W to 1, down 1 through C and Y to the galvanometer. It will return to the battery by way of Z, D, 3, B, and X. The current can then be rapidly reversed by raising and lowering N.



CHAPTER XVII.

RESISTANCE COILS.

APPARATUS 131.

230. Resistance Coils. Fig. 105. For experiments in resistance (See text-book), a set of standard resistances is necessary. There are many ways in which the resistances may be made; you can arrange them upon a long board, upon a rack, or wind the wires around spools. We generally speak of resistance coils. The Ohm is taken as the standard. If you use copper wire, you may take 9 ft. 9 in. of No. 30 insulated wire as your standard Ohm. You could, of course, take any other length of any size as your standard, but it will be best to make your coils with a certain number of Ohms resistance. If you have no No. 30 wire, you may use 39 ft. 1 in. of No. 24 insulated copper wire for 1 Ohm. (See wire tables in text-book.)



231. To avoid the magnetic effect (See resistance coils, in text-book), the wire should be measured off, then doubled, before winding it upon the spools. The wire may be held to the spool with paraffine. Fig. 105 shows how the doubled wire looks on the spool, a few turns only being shown. Do not use any nails or other iron in connection with the coils proper.

232. By making 4 coils having, respectively, 1, 2, 2, and 5 Ohms resistance, you will be able to use any number of Ohms from 1 to 10. These will be very handy in connection with a "Wheatstone's bridge" for comparing resistances. (See text-book for experiments). The coils should be mounted upon a base with proper binding-posts, so that one or more coils can be used at a time. (See App. 132.) For the 2-Ohm coil use, of course, twice as much of the same kind of wire as for the 1-Ohm coil.

APPARATUS 132.



233. Resistance Coils. Fig. 106. The construction of one coil is given in App. 131. To have the set of coils so that they can be easily used, place the spools upon a base which, in the model, is 8-1/2 x 4 x 7/8 in. The spools are 1-3/4 in. apart, center to center, and should be glued to the base. Fig. 106 is a plan of the apparatus. U, V, etc., are binding-posts like App. 46. The figures between them show how many Ohms resistance there are in the coil above. The coils A, B, C, D, and E are wound respectively for 1, 2, 2, 5 and 10 Ohms.

234. Connections. If you join a Wheatstone's bridge, for example, with U and V (Fig. 106), the resistance added will be but 1 Ohm; if you join with U and W, the coils A and B will be in the circuit and make 3 Ohms resistance; if V and X, 4 Ohms; if V and Y, 9 Ohms; if U and Z, the whole, or 20 Ohms.

APPARATUS 133.

235. Resistance Coils. For use in some experiments in comparing the resistance, diameter, lengths, etc., of wires (See text-book), it is very handy to have coils made a certain number of meters long. (The meter is a French unit of measure and represents 39.3705 of our inches). German-silver wire has a much greater resistance than copper wire of the same size and length.

(a) Make a coil (See App. 131 for method) containing 1 meter of No. 30 German-silver wire.

(b) Make a coil with 2 meters No. 30 German-silver wire.

(c) Make one with 2 meters of No. 28 German-silver wire.

(d) Make one with 20 meters of No. 30 copper wire.

The above wire must be insulated if it is to be wound upon spools. Bare wire may be arranged on boards or racks so that the current may not be short circuited.



CHAPTER XVIII.

APPARATUS FOR STATIC ELECTRICITY.

236. Static or Frictional Electricity. There are many interesting and instructive experiments in this branch of electricity. All that can be done here is to explain a few pieces of simple apparatus to show the presence of static electricity, it being taken for granted that you know how to produce it, and that you have some book of simple experiments.

237. Electroscopes are instruments for showing the presence of static electricity.

APPARATUS 134.

238. Thread Electroscope. A piece of ordinary thread may be used for this purpose. Tie one end of it to the back of a chair or other support.

APPARATUS 135.

239. Pith-Ball Electroscope. Fig. 107. The pith from elder, corn-stalk, milk-weed, etc., is very light and porous. When this is tied to the end of a silk thread, we get the pith-ball electroscope, so much talked about in nearly every text-book on physics. The upper end of the thread may be tied to any suitable support. Fig. 117 shows a book, lead pencil, and a small weight to hold the pencil steady. The thread is tied to one end of the pencil.



APPARATUS 136.

240. Support for Electroscopes, etc. Fig. 108. Glue or nail a spool, S, to a wooden base, B, measuring about 4 x 5 in. Wrap some paper around a 7 in. length of 1/4 in. dowel, D, to make it fit the hole in S. Wind one end of a wire, W, around the top end of D. To the outer end of W tie a silk thread, S T, on the lower end of which may be tied a piece of pith or material to serve as an electroscope.



APPARATUS 137.

241. Carbon Electroscope. Carbon will be found to make a most excellent electroscope, as it is light and a good conductor of electricity. Light an ordinary match and let it burn until it is charred through and through. The black substance remaining is carbon. Tie a small piece of the carbon, about 1/4 in. long, to one end of a silk thread, and support the thread as in Fig. 107 or 108.

APPARATUS 138.

242. Pivoted Electroscope. Fig. 109 and 110. Fold a piece of stiff paper double, then cut it into the shape shown. It should be about 3 in. long and 1 in. wide when opened out. A hole, B, about 1/2 in. in diameter should be cut in it while folded. A piece of paper, C, should be pasted to A, so that its top, where it is creased, will be about 1/8 in. above the top of A. The support consists of a pin, E, stuck through a cork, D. Balance the paper on the pin, which passes up through the hole, B. An electrified body brought near this apparatus will make it whirl around very decidedly.



APPARATUS 139.

243. Fancy Electroscope. Fig. 111. Fold a piece of stiff paper double, then cut out some fancy-shaped figure, as suggested, and draw the face, clothes, etc., to suit. This being folded through the center for cutting, it can be balanced upon a pin-point as explained in App. 138.



APPARATUS 140.

244. Box-Cover Electroscope. Fig. 112. A pasteboard box-cover, balanced upon a pin, makes a fairly good electroscope, although it is not nearly so sensitive as App. 138. The pin may be stuck in the upper end of the dowel, D, shown in Fig. 108.

APPARATUS 141.

245. Leaf Electroscope. Fig. 113. This is a very sensitive instrument, and can be used to tell the kind of static electricity on a body, as well as the mere presence of it. (See experiments in text-book.) The lamp chimney acts as a support for the leaves, L, and it protects them from currents of air. A tin box-cover, C, has a small hole punched through its center. Through this is pushed one end of a wire, W. This may be a hairpin, straightened. The upper end is bent over at right angles, after passing it through the hole. The lower end is bent as shown. On this horizontal part is fastened the leaf. These should be made of aluminum leaf, or of Dutch metal. The former will stand more rough handling than the latter. Goldleaf is used for very sensitive instruments. It is a little too delicate for unskilled hands.



246. To cut the aluminum leaf, place it between two pieces of paper, then cut paper and all into the desired shape. The piece should be about 3 in. long and 1 in. wide. Fold this across the middle, and stick it to the underside of the wire (Fig. 113). Saliva will make it adhere to the wire, if you have nothing better.

APPARATUS 142.

247. To Show Where a Charge of Static Electricity Resides. Fig. 114. This shows a tin baking-powder box placed upon a hot tumbler. A moist cotton thread is hung over the edge of the box. (See experiments in text-book.) The box will become charged by touching it with a charged body. The thread will show whether the charge resides upon the inside or upon the outside of the box.

APPARATUS 143.



248. Support for Electrified Combs. Fig. 115. In the study of static electricity, ordinary ebonite combs can be used to great advantage. A bent hairpin will serve as a cradle to support them. A silk thread may be tied to the wire, but a narrow silk ribbon is better than thread, as it will hold the comb steady.



CHAPTER XIX.

ELECTRIC MOTORS.

249. An Electric Motor is really a machine. If it be supplied with a proper current of electricity, its armature will revolve; and, if a pulley or wheel be fastened to the revolving shaft, a belt can be attached, and the motor made to do work. There are many kinds of motors, and many simple experiments which aid in understanding them. All that can be done here, however, is to show how to make simple motors. (See text-book for experiments.)

APPARATUS 144.

250. Electric Motor. Fig. 116, 117. Fig. 116 shows a plan or top view, and Fig. 117 shows a side view, with a part of the apparatus removed, for clearness.

The base, B, is 5 x 4 x 7/8 in. The upright, U, is 3-1/2 x 1-1/2 x 1/2 in., and is nailed or screwed to B. The binding-posts, X and Y are like App. 46. 4 is a screw binding-post.

251. The Field-Magnets, as the large electro-magnets on a motor are called, are made of 5/16 machine-bolts, 2-1/2 in. long. The washers are 1-1/2 in. apart inside. (See App. 88 for full directions.) The bolt cores are 2 in. apart, center to center. (See App. 89.) The tin yoke, D, is made like App. 71, and it is fastened to the base, like App. 90. The hole for the screw, however, is made a little to one side of the center, so that a dent can be made at the center for the bottom of the shaft, 8, to turn in. Make the dent with a center punch. The yoke is fastened to B, so that one edge of it is 1-1/2 in. from the back edge of B. (Fig. 116).

252. The Armature, A, is made of 6 or 8 thicknesses of tin, 2-1/2 in. long and 3/4 wide. (See App. 71.) In its center is punched or drilled a 1/4 in. hole, so that it can be slipped onto the 1/4 in. "sink-bolt," 8. If you have taps you can make the hole a little smaller than 1/4 in., and thread it so that it will screw onto 8. A must be heavy enough to revolve a few times when once started. It is pinched between two nuts, 9 and 11, so that it just clears the poles when it turns. (See App. 145 for another form of armature.)



253. The shaft or axle, 8, is made of a "sink-bolt" that is 3 in. long and 1/4 in. in diameter. These sink-bolts are threaded over their entire length, and are furnished with two nuts, 9 and 11, Fig. 117. File or grind the end of 8 to a point, so that it will turn easily in a dent made for it in the yoke, D, or in a dent made in another piece of tin fastened over the yoke. The shaft is held in a vertical position by the arm, C.

254. The Arm, C, is made of 2 or 3 thicknesses of tin. It is 3 x 3/4 in.; it has in one end a hole for the shaft to revolve in easily, and in its other end a slot is cut. A screw-eye and bur are used to hold C to the upright, U. By this means the shaft can be moved and regulated as to position.



255. The Commutator, 9, (Fig. 117), is made of one of the nuts furnished with the shaft. Two of its corners are filed or ground off, so that it has the shape shown at the right, in Fig. 117. The copper wire, 10, rubs against 9, as the pointed part of it comes around. 10 is really a "brush," and carries the current into 9 at the right time.

256. Connections. Join the two inside ends (Sec. 123) of the coils to 4. The outside end of 2 is joined to X; the outside end, 7, of the other coil, 6, is carried up under or around the screw-eye, S I, and then its bare end reaches out and gently scrapes against the top of the shaft, 8. The wire, 10, leads from Y to the back of the base, where it is carried up to a screw, 12, which holds it to U. Its bare end reaches out to gently scrape against the commutator, 9, when it swings around. This wire, 10, should not press against 9 during the entire revolution.

257. Adjustment. Suppose the current enters at X. When the "brush," 10, presses against the commutator, 9, the current passes through X, 1, 2, 3, 4, 5, 6, 7, down 8 to 9, and out through 10 to Y. (The current, of course, goes down into D and into the bolt-cores also; but it can go no farther, if the coils are properly insulated, and A is not allowed to touch the cores. It is better to have the end of the shaft rest upon a piece of glass, having a slight depression made with a file, or in a dent made in tin which rests upon wood, the tin having no connection with D.) If 10 should continue to press against 9, the current would continue to pass, and A would be held firmly in place, directly over 2 and 6, and, of course, the shaft could not revolve. If, however, the brush leaves 9 (See plan of 9 at side of Fig. 117), just as A gets over the coils, or an instant before it gets there, the weight of A will carry it beyond the coils. No current should pass again, until A is at least at right angles to a line drawn through the center of the coils. If the current again passes, the ends of A will be attracted by the bolt-cores.

In other words, the current should pass a little less than one-half of the time, and this is divided into two parts. Suppose you start A with your finger; the current should be shut off automatically just before the center of A gets over the center of the bolt-cores. A makes 1/4 of a revolution without current, and just after it gets beyond this, the current passes for nearly 1/4 of a revolution, which brings the ends over the poles again. The next 1/4 of a turn it has no current, because the flat side of 9 is opposite the brush, 10, as during the first 1/4. The last 1/4 the current passes again. The exact position of the commutator will depend upon the way you arrange the brush. The positions of 9 and 10 can be found by trial, so that the circuit will be promptly opened and closed at the proper moment. Start the motor by turning the armature.

258. Batteries. The amount of power needed will depend upon how well you make the motor. One cell of App. 3 or 4 will run a well made one, but it is better to use 2 cells. Join the wires to X and Y.



APPARATUS 145.

259. Armature for Motors. Fig. 118 shows another form of armature that may be used for small motors like App. 144; in fact, you may find that this form is easier to make than that of App. 144. M is a 5/16 machine screw, 1-1/2 in. long, 9 being the nut furnished with it. 9 is filed as explained in Sec. 255, and forms the commutator. C is the arm (Sec. 254). A is the armature (Sec. 252). A is held firmly in place between the spool, E, and 9. S is a set-screw which passes through E, and holds the piece of 1/4 in. dowel, F, in place. N is a needle-point fastened in the end of F. N revolves in a dent made in a piece of tin, H, which rests upon a wooden strip, G. G is cut away on its underside, so that it will straddle the yoke, D, Fig. 117; it is nailed to the base. This is given as a suggestion. By making F a little longer, N can turn in a dent made in the yoke, below G.

260. Adjustments. M, being 5/16 in. in diameter, will screw solidly into the hole in E. Place 9 upon it first, then A, and screw it about 1/2 way into E. 9 will serve as a lock-nut by turning it so that it will pinch A and hold it firmly against the top of E. F should reach half way into E. Put N in place after you have H and G arranged. You can then cut the upper end of F at such a place that it will bring A about 1/8 in. from the top of the magnet-cores. Paper wrapped around F will make a good fit in E. The current should enter M and leave 9, as fully explained in App. 144. (See Sec. 257).

APPARATUS 146.

261. Electric Motor. Fig. 119, 120, 121, 122. Fig. 119 shows a front view, and Fig. 120 a side view of the whole motor. Fig. 121 shows the part that revolves, and includes the shaft, armature and commutator. Fig. 122 shows a section of the commutator. All the dimensions are taken from a model. You can modify the size to suit.

262. Wood-work. The base is 7 x 5 x 7/8 in. The uprights, U, are 3-1/2 x 1 x 3/4 in. They are screwed or nailed to the base from below, their 1-in. sides being towards you in Fig. 119. They are 4-1/4 in. apart, inside, in this model. The piece, A, is 2-1/2 x 7/8 x 5/8 in., and is cut away on the underside to straddle the yoke. Fig. 118 is a suggestion as to its shape. A is screwed or nailed to B.

263. Tin-work. The horizontal arm, T, is made of 3 thicknesses, and holds the shaft in a vertical position. T is 6-1/4 x 3/4. In its ends are slots, and in its center is a hole so that the 1/4 in. shaft can revolve easily, but not too loosely. The slots allow an adjustment, the screws, S, holding T to U. The shaft rests in a dent made in a piece of tin which is tacked to A. The yokes are elsewhere described.



264. Field-Magnets. In this model they were made of 5/16 bolts, 2 in. long, placed 2 in. apart center to center. The washers are 1-1/8 in. apart inside. (See App. 88 for full directions.) App. 89 and 71 should be studied. Except in size, they are made as in App. 144. They have 8 layers of No. 24 or 25 wire.

265. The Armature, Fig. 121, on this style of motor consists of a regular horseshoe electro-magnet, made in the same general way as the field-magnets. The electro-magnets, 12 and 16, are smaller, however, than the field-magnets. The cores are 1/4 in. stove-bolts, 1-1/4 in. long under the head. They are placed 2 in. apart, center to center. They are insulated and wound as fully explained in App. 88. These 1/4 in. bolts require a change in your winder. (See App. 147 for this.) If you wish to use 5/16 bolts, you may use the same axle for your winder as before. The washers are 5/8 in. apart, inside. The cores are wound with 4 or 6 layers of No. 24 or 25 wire. This makes them about 3/4 in. in diameter. They are held in a tin yoke, 14, made of 5 or 6 thicknesses of tin. 14 is 3 x 3/4 in., and has 3 holes punched in it. The two outside holes are 2 in. apart. Through these pass the bolts, which are held firmly by the 2 nuts. The shaft, S B, is a sink-bolt, 3 in. long, and 1/4 in. in diameter. (See Sec. 253.) The inside ends (Sec. 123) of the coils should be firmly twisted together or held under the top nuts to make a good connection between them.



266. The Commutator is in two parts, which must be insulated from each other. The 2 sections are made out of thin tin or copper in the shape of an inverted T, as shown at 10, Fig. 121. The arms of the T are about 3/8 in. wide, the horizontal ones reaching about half around the spool, E. The vertical arm reaches over the top of E, and is held down by a small screw, J. The sections, 10, must not touch the shaft. The outside wires (Sec. 123) of 12 and 16 are fastened under these screws, J, and they must not touch the shaft. Bend the tin sections so that they will be as nearly round as possible. The spool, E, has been sawed off so that it will go between the field-magnets. Wind paper around the shaft to make it fit solidly into E. S is a small screw that holds E in place, if the paper does not hold it tight enough.



Fig. 122 shows a section of the spool and tin sections with the brushes pressing against them. The sections do not touch each other, and the brushes touch opposite sections. It is evident, then, that the current must pass through the coils 12 and 16 in order to get from one section of the commutator to the other, provided you have no short circuits through the shaft or elsewhere. The slots in the commutator must be directly under the center line of the yoke, 14, as seen in Fig. 121.

267. The brushes, 9 and 19, Fig. 120, are made of very thin tin or copper. They are cut to the shape shown, the narrow part being about 1/8 in. wide, and long enough to reach at least to the center-line of the apparatus. The foot, or bottom part of the brushes, should be about 1-1/4 x 3/4 in. These are used to fasten them to the base and to make connections. If you have no thin metal for brushes, use copper wires, and arrange them so that they will press gently against the commutator.

268. Connections. The inside ends (Sec. 123) of the field-magnets are held at 4. The outside end of coil 2 is joined to X, and that of coil 6 to 8, the foot of the brush which presses against 10. The section, 10, of the commutator is joined to 11, the outside end of coil 12, its inside end being fastened to the inside end of coil, 16, either by twisting them together, or by fastening them under the top nuts of the armature yoke, 14. The outside end of coil 16 is joined to the other commutator section, 18. The brush, 19, completes the circuit. In the foot of 19 is the binding-post, Y.

If the current enters at X, it will pass through 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and out at Y, provided 10 and 18 are in contact with 9 and 19. Be careful not to have any short circuits. If, for example, the wire 7 touches 4, or if 3 touches 8, or if the wires 11 and 17 touch the shaft, your current will not pass where you expect, and you will have trouble.

269. Adjustment. The armature cores should just clear the poles of the field-magnets as they turn. This must be regulated by the thickness of A and the position of the nuts on the shaft, S B. The slots in the commutator must be under the center of the yoke, 14. The brushes, 9 and 19, must touch 10 and 18, but not so hard that they will stop the motor. Wire brushes are more easily adjusted than tin or sheet-copper ones. The tin arm, T, must hold the shaft properly. The point of the shaft must allow it to turn easily. The motor will turn clockwise if the attachments are made as shown. Use 1 or 2 good bichromate cells, like App. 3 or 4.

270. Operation. The current will pass through the field-coils in the same direction, as long as the battery wires are not changed. The current is reversed in the armature-coils every time the brushes change from one section to the other of the commutator; that is, it flows in one direction during one-half of a revolution, and in the opposite direction during the other half. This reverses the poles of the armature-magnets every 1/2 revolution. (See text-book for full explanations and for simple experiments with electric motors.)

APPARATUS 147.

271. Attachment for Winder. In winding small electro-magnets for armature, etc., in which cores are used that are not 5/16 in. in diameter, your winder will have to be slightly changed. Its 5/16 stove-bolt will have to be removed, and a 1/4 in. one put in instead. This may be done by making a handle for the 1/4 in. bolt. To keep this from wobbling in the 5/16 hole, wind stiff paper around the bolt until it fits quite tightly. The whole winder is explained as App. 93.



CHAPTER XX.

ODDS AND ENDS.

APPARATUS 148.

272. Graduated Circles. Fig. 123. For compasses (App. 67), and for use in connection with tangent galvanometers (App. 116), a graduated circle is necessary. Fig. 123 is a reduced drawing from an original that is 4 in. in diameter. The long lines are 10 degrees apart, the smallest divisions shown being 5 degrees apart. Single degrees can be determined with considerable accuracy with the eye.



To divide the circle. Divide the circumference into 4 equal parts; these will be 90 degrees from each other, there being 360 degrees in every circle. Divide each quarter into nine equal parts with a pair of dividers; these will be for the long lines, 10 degrees apart. Divide each of these into two equal parts. If you are used to drawing, you can divide the circle still more, but 5-degree divisions will do.

APPARATUS 149.

273. Adjustable Table. Fig. 124. A table that can be raised or lowered is useful. The one shown at D, Fig. 124, is used for the galvanometer of App. 117. The dimensions are given in the figure. The upright piece, U, is fastened to D with brass screws, not with nails, as these would affect the needle. It is placed at one side of D so that the compass needle placed in the center of D will also be in the center of the wire coils when used in App. 117. The table is fastened in any position by a screw-eye, S I, which presses a copper washer, W, against U. S I works through a narrow slot, S, and screws into the back of the galvanometer. By making S longer, the table may be used for other laboratory purposes, if it is joined with some other form of standard.



APPARATUS 150.

274. Glue Pot. If you have occasion to use glue, you can make a good glue pot out of 2 tin cans, one being placed inside the other. Put 1/4 teacupful of glue in the inside can. If you have time, cover it with cold water, and let it soften. If you are in a hurry, cover it with hot water. Set this inside can into the other, in which you have boiling water. Do not let the water boil over. The solder will not melt from ordinary tomato cans, if you keep water in them. Thin the glue with a little hot water until it drips from the brush in drops. Have the glue hot and fairly thin, and apply quickly. Hold the pieces of wood together by pressure until the glue hardens.

APPARATUS 151.

275. Paraffine Paper and Cardboard are extremely useful for insulating purposes. The paraffine used in candles will do, if you cannot get it in block form. While ordinary paper will do for simple apparatus to wind about coils, etc., you will find that paraffine paper can be handled very rapidly. To melt the paraffine you should use a double boiler, or one made of a shallow basin set in a pan of water. The water should be boiled. This will melt the paraffine in the basin. Strips of paper just passed through the melted paraffine will become soaked, and the paraffine will quickly harden in the air. Allow thick cardboard to soak for a minute or two, to drive out all the air. This makes excellent washers for electro-magnets. (See Sec. 119.) To make one piece of this paper stick to another, merely pass a clean hot nail over the two where they lap. To hold coils of wire together, or to wooden bases, use a few drops of paraffine applied with a large hot nail.

276. Caution. Do not heat paraffine directly upon the fire or over a burner, unless you watch it constantly. It will burn if its temperature is raised too much. It is better to heat it with steam, as you do glue.

APPARATUS 152.

277. Battery Jars. For small cells, use glass tumblers. Ordinary glass fruit jars are good. Even earthen bowls may be used, and for large cells—if you have nothing better—you can use small earthen crocks or jars.

278. Glass Bottles can be cut off so that they will make excellent jars. If you have thin bottles, you can cut them with strong cord. Tie one end of the cord, which should be 5 or 6 feet long, to a door knob or to a solid post. Tie the other end around your body. Make one complete turn of the cord around the bottle where you wish to cut it; draw the cord tight by stepping back, and with both hands draw the bottle back and forth vigorously many times, so that the cord will rub it hard and make it very hot. Do not let the cord move lengthwise upon the bottle. This will make a circle around the bottle that is very hot. Immediately plunge the bottle into cold water, the colder the better. Use ice-water, if you have it. If you produce heat enough, the bottle should crack all the way around very neatly. File off any sharp corners and edges with a wet file.

279. A hot iron can be used with success to cut off a bottle. File a deep groove first, hold the red-hot iron first on one side of file mark and then on the other to start the crack. You can lead the crack wherever you wish by keeping the iron about 1/8 in. ahead of it.

280. A small gas-flame will be much better than a hot iron, and you may easily use it, if you have glass tubing, rubber tubing, etc., in your shop. Draw out the glass so that the gas will burn in a fine needle-like flame about 1 in. long. Keep the point of the flame about 1/4 in. ahead of the crack. The glass tube should be held in a rubber tube connected with the gas pipe.



CHAPTER XXI.

TOOLS AND MATERIALS.

281. Your Workshop. If possible, keep all your work, tools and apparatus in one room, and lock the door when you leave.

The work-bench may be made of an old kitchen table, or of a strong, large box. The tool chest may be made of any clean box about the size of a soap box. Shelves can be made by setting soap or starch boxes on their sides, one above the other.

282. The tools needed are generally mentioned in the proper places, under the directions for construction. It is better to buy your tools as required, than to buy too many at once, some of which you may not need. If you have absolutely no tools, not even a saw or hammer, you will be obliged to buy or borrow, although a great deal can be done with a good knife. Do not be satisfied with rough-looking pieces of apparatus.

There are a few important tools needed for this work. While substitutes can be found for most of them, the boy who has access to a wood-working bench and tools will be able to do better and more rapid work than the boy who has no such tools.

283. List of tools. The following tools are needed, if rapid, accurate work is desired:

(1.) Lead pencil. (2.) A rule, divided into sixteenths for measuring. A straight foot rule will do,—cost one cent. (3.) Steel point for scratching lines on tin and copper. A stout needle-point is just the thing. (4.) An awl for making holes in wood; one that is a little less than 1/8 in. in diameter is best. (See App. 25.) (5.) A try-square with a 6 in. blade, so that you can mark out your apparatus with square corners. You can use a square-cornered box or piece of pasteboard, if you have no try-square. (6.) Chisels are very useful, but you can do wonders with a good sharp knife. (7.) Screw-driver. Do not use a good knife-blade for a screw-driver. (8.) A saw, one with teeth that are not too coarse is to be preferred. (9.) A plane is extremely useful to make your wood-work smooth and neat; but a great deal can be done with the sharp edges of broken glass, followed by a good rubbing with fine sand-paper. (10.) A brace and a set of bits may be needed in 2 or 3 cases, but nearly all of the holes can be made as in App. 25. (11.) Punches for sheet-tin, etc., will save much time. (See App. 26, 27.) For small holes in binding-posts, etc., use a flat-ended punch, 1/8 in. in diameter. You should have one 1/4 or 5/16 in. in diameter, if you make your yokes, armatures, etc., as in Chapter VIII. A blacksmith will help you out with this. (12.) A center-punch or sharp-pointed punch for making dents in metal. A sharp-pointed wire nail will do for tin and copper. (13.) Files for metal. (14.) Some sort of a vice or clamp. (See App. 79, 80.) (15.) Shears for cutting sheet-tin, etc. A pair of old shears will do. (16.) An anvil or piece of old iron that may be used to hammer on to flatten tin, etc. An old flat-iron makes a good anvil. (17.) Hammer.

The small hollow handle tool sets are very handy, and they contain small chisels, awls, screw-driver, etc. These sets cost from 50 cents up.

284. Materials. For wood you will find the sides and ends of clean soap or starch boxes about the right thickness; they are fairly smooth to begin with. For thin wood use cigar boxes. The pieces from old boxes should be removed with care, and saved in one place, which may be called your lumber yard. All nails should be removed with a claw-hammer. Look out for nails when using a saw, plane or other edged tool. (See Sec. 297.) The edges of bases, etc., may be bevelled as shown in Fig. 95. This is not necessary, but it adds greatly to the appearance.

285. Screw-Eyes. Brass screw-eyes, with copper burs, make excellent binding-posts. (App. 45, 46.) Those that are 3/8 in. in diameter inside the circle are about right. These are about 1-1/4 in. long in all, with a 1/2 in. thread.

286. Copper Burs, such as are used with rivets, are very handy. The size that is 1/2 in. in diameter, with a 1/8 in. hole, is good.



287. Copper Wire. This can be bought at an electrician's. The only trouble, however, in buying small quantities is that you may have to pay a large price in proportion. If you get it on 1/2 lb. spools you can handle it much better (see App. 23) than you can if you have it in a tangle. It is well to have 1/2 lb. of No. 24 or 25 for electro-magnets, current-detectors, etc., etc. 1/2 lb. of No. 30 will not be too much, if you make induction coils. If you handle your wire carefully, single cotton-covered will do. Double cotton-covered is better than single, but it costs more. Be careful not to injure the covering. (See below for splicing wire.) Look out for broken wire.

288. Splicing Wire. Fig. 125. Do not simply touch two wires together and imagine that you have a good connection; a mere twist is not sufficient. Clean the ends of old wire thoroughly with a file or knife-blade, and join them as shown in Fig. 125.

289. Copper. Sheet-copper can be purchased at a tinsmith's or at a hardware store. Electricians usually have a thin variety of copper called brush copper, which makes good battery-plates, binding-posts, etc. You can cut this thin copper with an ordinary pair of shears.

290. Iron. For thin sheet-iron, nothing is better than sheet-tin. (See tin.) Hoop iron is thicker than tin, and makes good yokes, etc. In many cases, ordinary nails may be used where a magnetic substance is needed. Annealed iron wire is extremely soft. (See text-book for experiments with steel and iron.)

291. Steel. Old files, watch-springs, clock-springs, corset-steels, knitting-needles, harness-needles, hack-saw blades, sewing-needles, etc., are generally made of a good quality of steel.

292. Zinc, in the sheet form, can be bought at a hardware store. For a few cents you can get quite a large piece. Get the thick pieces for heavy battery-plates of an electrician. You do not need anything that is thicker than 1/8 in. The zinc rods are usually amalgamated.

293. Lead can be bought at a plumber's, tinsmith's, or hardware store. You may want some for a storage cell.

294. Nails. Wire nails are best for light work. Get an assortment from 1/2 in. long up to 1-1/2 in.

295. Screws. It is better to use brass screws around electrical apparatus. For the small work, for binding-posts, etc., use 5/8 No. 5. Another handy size is No. 7, from 3/4 to 1-1/4 in. long. The round-headed screws are best, unless you want to countersink them.

296. Tin. This is really thin sheet-iron, covered with tin. Save up tomato-cans, cracker-boxes, condensed-milk cans, etc. The cracker-boxes are just as good as sheet-tin, as the pieces are large and clean. You can remove the solder from cans by heating them in the kitchen fire. Knock out the bottoms with a poker when the solder gets soft. Clean the tin with sand-paper.