Chapter 4

SWITCHES AND CUT-OUTS.

_52. Switches, Cut-Outs._ Where apparatus is to be used frequently, such as for telephone and telegraph lines, it pays to make your switches, etc., carefully. The use of these switches, etc., will be shown in the proper place. Their construction only will be given here.

APPARATUS 36.

_53. Cut-Out._ Fig. 22. Details. X, Y, and Z represent 3 binding-posts like App. 42. These are fastened to a wooden base that is about 3 x 5 x 3/4. The ends of the wires shown come from and go to the other pieces of apparatus. Q shows a stout wire or strip of 2 or 3 thicknesses of tin. Suppose we have an apparatus, as, for example, an electric bell, which we want to have ring when someone at a distance desires to call us. If we use a telephone or telegraph instrument we shall want to cut the bell out of the circuit as soon as we hear the call and are ready to talk. Suppose the current comes to us through the wire, A, Fig. 22. It can pass by the wire, C, through the bell and back to X. If we wanted simply to have the bell ring, the current could pass directly from X into the earth, or over a return wire back to the push-button at our friend's house. If, however, we are to use some other instrument, by lifting the end of Q out of X and pushing it into Y, the bell will be cut out, and the current can pass on wherever we need it.

APPARATUS 37.

_54. Cut-Out._ Fig. 23. The main features of this are like those of App. 36. The three binding-posts are like App. 46. Instead of a band of metal to change connections, as Q in App. 36, a stout copper wire is used. This can be easily changed from one of the upper binding-posts to the other, thereby throwing in or cutting out any piece of apparatus joined with the upper connectors.

APPARATUS 38.

_55. Switch._ Fig. 24. This simple switch has but one contact point, D, which is a screw-head. This switch may be used anywhere in the circuit by simply cutting the wire carrying the current, and joining the ends of the wire to the binding-posts X and Y. The metal strip, E, is made of 2 or 3 thicknesses of tin. It is 5/8 in. wide and about 5 in. long, and presses down upon D, when swung to the left, thus closing the circuit. The short metal strips shown are 5/8 x 1-1/4 in. The upper strip is joined to the end of E by a coiled copper wire, C W. (See App. 50.) If the current enters by the wire, A, it will pass through C W, E, D and out at B. The strip E is pivoted at F by a small screw. The base may be 3 or 4 x 5 x 7/8 in.

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.

Details. The base of the one shown in Fig. 25 is 4 x 5 x 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 brass 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 pass 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 pass 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 pass 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 pass.