How Two Boys Made Their Own Electrical Apparatus Part 12

179. The Base should be about 7 5 7/8 in. Fasten three bent bra.s.s or copper strips to the base with bra.s.s 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 pa.s.ses 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.

[Ill.u.s.tration: Fig. 89.]

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 4 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.

[Ill.u.s.tration: Fig. 90.]

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, pa.s.s 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 1/4 1/4 in., to be put under G, one being on each side of the coil. Make holes through the base, pa.s.s 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 pa.s.s through five turns of wire; if connected with Y and Z, it will pa.s.s through ten turns; if with X and Z, the current will pa.s.s 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 pa.s.ses. 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 pa.s.s 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 s.c.r.e.w.i.n.g B to A the base is made thick enough to take the screws for binding-posts. The base proper, A, is 8-1/2 5 1/2 in. If you make this of 7/8 in. stuff, you will not need B.

The Back, C, is 10 8-1/2 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 circ.u.mferences 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.

[Ill.u.s.tration: Fig. 91.]

The Spools, F, are glued to C. If you have bra.s.s 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 coa.r.s.e 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 pa.s.sing 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 pa.s.sing 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 gla.s.s-covered compa.s.s. (See App. 67 for details.) The needle should not be over 1 in. long.

CHAPTER XIV.

TELEGRAPH KEYS AND SOUNDERS.

APPARATUS 118.

[Ill.u.s.tration: Fig. 92.]

_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-b.u.t.ton 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 pa.s.s 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-b.u.t.ton 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 4 7/8 in. The key, C, is made of two thicknesses of tin. It is made into a strip 5-1/2 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.

[Ill.u.s.tration: Fig. 93.]

The Strap, D, is made of tin. It is 4 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 pa.s.s 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 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 pa.s.s 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 4 3/4 in., and the back, A, is 6 5 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 pa.s.s around it near the poles and at the curved part. The wire should pa.s.s 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.

[Ill.u.s.tration: Fig. 94.]

197. The Armature, C, is made of a narrow piece of thin iron, about 5-1/2 1/4 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 pa.s.ses, 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 pa.s.ses, because the residual magnetism would keep C from dropping back into place. To adjust the armature, pa.s.s 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.