How Two Boys Made Their Own Electrical Apparatus Part 14

As the bolts were no longer magnets, F sprang right back to I, the current pa.s.sed 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.

[Ill.u.s.tration: Fig. 101.]

_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 ill.u.s.trations. 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 5 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 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 pa.s.ses 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 pa.s.sed through a push-b.u.t.ton.

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.)

[Ill.u.s.tration: Fig. 102.]

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 (-- 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 pa.s.s 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 pa.s.s 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 pa.s.sing 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.

[Ill.u.s.tration: Fig. 103.]

APPARATUS 129.

_225. Current Reverser._ Fig. 103. The base is 5 4 7/8 in. To this are fastened four metal straps, A, B, C, and D. These may be made of bra.s.s, 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 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 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 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 pa.s.sed 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 pa.s.s 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 pa.s.s 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 pa.s.s out of Y or Z to the galvanometer at will by pressing down A or B.

APPARATUS 130.

[Ill.u.s.tration: Fig. 104.]

_228. Current Reverser._ Fig. 104. The wooden base is 7 5 7/8 in. To this are fastened two bra.s.s or tin straps, C and D, 5 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 3 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, pa.s.s 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 pa.s.s along A to 4, before it can find a chance to escape. It will pa.s.s 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 pa.s.s 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.)

[Ill.u.s.tration: Fig. 105.]

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.