How Two Boys Made Their Own Electrical Apparatus Part 4

APPARATUS 39.

_56. Switch._ Fig. 25. By increasing the number of contact points and the wires leading from them, a switch may be made to throw in one or more pieces of apparatus. This variety of switch is useful in connection with resistance coils (Index). By joining the ends of the coils with the points 1, 2, 3, etc., more or less resistance can be easily thrown in by simply swinging the lever, E, around to the left or right. The uses of this will be again referred to.

[Ill.u.s.tration: Fig. 25.]

Details. The base of the one shown in Fig. 25 is 4 5 7/8 in. thick.

The switch, E, is a band of 2 thicknesses of tin 5/8 in. wide. It is pivoted at F with a screw. To the end of E is fastened a copper wire, which leads to the upper binding-post, X (App. 46). The apparatus has 5 contact points, marked 1, 2, 3, etc. These consist of bra.s.s screws and copper washers. With F as a center draw the arc of a circle that has a radius of 4 in. Place the screws 1, 2, etc., along this arc, and about 5/8 in. apart, center to center; that is, the screws are all 4 in. from F, and are, therefore, in the form of a curve.

The last screw forms a part of the binding-post, Y. Suppose 4 pieces of apparatus, marked A, B, C, and D, be connected with 1, 2, etc., as shown. These may be, for example, coils of wire to be used as resistance coils. If the current enters at X, it will pa.s.s along at E and be ready to leave at Y, as soon as E touches one of the contact points. If E be placed upon 1, the current will be obliged to pa.s.s through all of the coils, A, B, etc., before it can get to Y. In this case the resistance will be greatest. If E be now moved on to 2, only A will be cut out, and the total resistance reduced. By placing E upon 4, but one coil, D, will be in the circuit. When E is upon 5 the current will pa.s.s through the switch with practically no resistance. This is the principle upon which current regulators work. (Study resistance in text-book.) When E is in the position shown in Fig. 25 no current can pa.s.s.

CHAPTER V.

BINDING-POSTS AND CONNECTORS.

_57. Binding-Posts_ are used to make connections between two pieces of apparatus, between two or more wires, between a wire and any apparatus, etc., etc. They are used simply for convenience, so that the wires can be quickly fastened or unfastened to the apparatus. There are many ways of making them at home. The following forms will be found useful and practical. Although some that are given are really connectors instead of binding-posts, we shall give them the general name of binding-posts.

APPARATUS 40.

_58. Binding-Post._ About the simplest form is a screw, or a nail with a flat head. The bare wire may be placed under the head of the screw or nail before forcing it entirely into the wood. This will keep the end of the wire in place, and another wire may be joined electrically to the first by merely touching it to the screw-head, or by placing it under the screw-head.

APPARATUS 41.

_59. Binding-Post._ Fig. 26. This consists of a screw and a copper washer or "bur." The screw is a "round-headed bra.s.s" one, 5/8 in. long, number 5 or 7. The copper burs are No. 8, and fit nicely around the screws. By using 2 burs instead of 1, several wires may be easily joined together at one point. Sc.r.a.pe the covering from the ends of the wires, and place them between the burs.

[Ill.u.s.tration: Fig. 26.]

APPARATUS 42.

_60. Binding-Post._ Fig. 27. A coiled spring serves very well as a connector. One end should be fastened to the apparatus, as shown, by clamping it under a screw-head. The other end of the coil should be pulled out a little, away from the other turns, so that you can stretch the spring in order to put the bare ends of wires between the turns. Any number of wires placed between these turns will be pinched and electrically connected. The coil should be about 1/2 in. long and less than 1/2 in. in diameter. You can make a coil by tightly wrapping stiff iron wire around a pencil. The steel wire springs taken from old window-shades are excellent for this purpose. They may be cut into lengths with tinner's shears.

APPARATUS 43.

_61. Binding-Post._ Fig. 28. Two copper or tin strips fastened at one end by a screw, the upper strip being bent a little at one end, make a connector that is useful for some purposes, where you want to make and break the connection frequently. The bare end of the wire which belongs to the apparatus is fastened under the screw-head. The outside wire, or wires, to be connected are pushed between the strips of metal. Another way is to fasten the outside wire to a strip of metal about 1/2 in.

wide, and then push this between the strips shown in the figure. The strips shown should be about 3/4 in. wide and 1-1/4 in. long.

[Ill.u.s.tration: Fig. 27.]

[Ill.u.s.tration: Fig. 28.]

[Ill.u.s.tration: Fig. 29.]

APPARATUS 44.

_62. Binding-Post._ Fig. 29. A combination made between App. 42 and 43 does well. Fasten a metal strip, 3/4 in. 1-1/4 in., to the apparatus by means of a screw. The apparatus wire should be fastened under the screw-head. A short length of spring may be pushed upon the upright part of the strip, as shown. Into this you can quickly fasten the outside wires.

APPARATUS 45.

_63. Binding-Post._ Fig. 30. This makes a very simple and practical binding-post for home-made apparatus. It consists of a screw-eye, preferably of bra.s.s. The circle or eye should be about 3/8 or 1/2 in. in diameter. The thread on such a screw-eye will be about 1/2 in. long. Two copper burs are used to pinch the wires.

APPARATUS 46.

_64. Binding-Post._ Fig. 31. This consists of a screw, screw-eye, bur and a metal strip, 3/4 1-1/4 in. The apparatus wire should be fastened under the screw-head. Any outside wires which are to be joined to the apparatus should be clamped under the bur by turning the screw-eye. A small hole should be made in the wood before putting in the screw-eye.

(See App. 25.) Do not turn the screw-eye too hard, or you will spoil the thread made in the wood.

[Ill.u.s.tration: Fig. 30.]

[Ill.u.s.tration: Fig. 31.]

[Ill.u.s.tration: Fig. 32.]

APPARATUS 47.

_65. Binding-Post._ Fig. 32. The size of the bolt used in this form of binding-post will depend somewhat upon the thickness of the base of the apparatus. In general, a 3/4 or 7/8 in. base should be used where screws or screw-eyes are necessary. With this kind (Fig. 32) a thin base can be used. The head is shown counter-sunk into the bottom of the base.

This is not necessary, provided at least 3 heads are placed far enough apart to form legs for the apparatus to stand on. Strips of wood may be nailed upon the underside of the base to make room for the heads in case they are not used as legs. The wires should be pinched between the nut and the copper bur shown. If the bolt is too large for a bur, an iron washer may be used. A washer may be made of tin, or two nuts may be used.

APPARATUS 48.

_66. Binding-Post._ Fig. 33. This is a suggestion for a combination of App. 44 and 47. It is useful in school apparatus. Wires may be permanently fastened on the right, under the nut, and a spring, as in App. 44, may be slipped on the metal strip at the left, which is held under the head of the bolt.

[Ill.u.s.tration: Fig. 33.]

APPARATUS 49.

_67. Mercury Connector._ A cup of mercury may be used as a connector.

Make a small hole about 1/4 in. in diameter and depth, in a piece of wood, and place 2 or 3 drops of mercury in this. The ends of wires dipped in this will be electrically connected.

APPARATUS 50.

[Ill.u.s.tration: Fig. 34.]

_68. Connector._ Fig. 34. This shows how a wire may be fastened to one end of a short strip of tin. At the other end of the strip a slot is cut. This may straddle the body of a screw, or when left plain may be used to slip between the two metal strips shown in App. 43.