Читать книгу Harper's Electricity Book for Boys - Joseph H. Adams - Страница 15

A table-jack switch is a most convenient piece of apparatus where several lines of bells, alarms, or telephone circuits are to be switched on and off.

The single table-jack switch, shown in Fig. 13, is made of a hard-wood block three-quarters of an inch thick, five inches wide, and seven inches long. It is to be smoothed and varnished, or given several coats of shellac. At the four corners small holes are made to receive slim screws, and at one end of the block five short metal plates are screwed fast, with the heads of the screws countersunk, so that they will be flush with the top of the plates. These small plates should be half an inch wide and one inch long, and may be of brass, copper, or tin. But if they are of tin the plates are made of a longer strip tacked to the board and then bent over, as shown at A in Fig. 14. They will therefore form short springs, the upper parts of which will rest against the long spring-arms. From spring brass or copper five arms are to be cut and shaped, as shown in Fig. 13. Holes are made at one end of each, and others again two inches from these, through which to pass screws.

Screw-eyes are passed through copper washers and the end holes in the strips, and then screwed into the wood plate. These will act as binding-posts, while the second line of screws will hold the plates down to the base. The arms should be bent, so that when the screws are driven down the lower edge will press on the small plates under them.

The outlet wires are attached to the binding-posts at the head of the block, and the plug (A in Fig. 13) is inserted between the arm and plate at the foot, so that contact and connection are made. This plug is a small plate of metal to which the end of a flexible wire is made fast. It should be of copper or brass, but for light work a strip of tin may be bent over with the wire caught between the plates and a copper tack passed through the sides and riveted, as shown at B in Fig. 14.

A double jack-switch (Fig. 15) is made on the same general plan as the single, but it has no binding-posts. A block of the same size is used, and two rows of short plates are made fast at each end. The arms are made with two screw-holes near the middle, as shown in Fig. 15, and through these holes screws are driven to hold the arms down to the base. Several plugs are used for each end, so that the in and out lines can be shifted, and from one to four lines used at a time.

TABLE-JACK SWITCHES

A convenient slip-switch for single or double line work is shown in Fig. 16. This consists of a small wooden base, on which a brass arm and handle are screwed fast and connected with a binding-post (A in Fig. 16). A slip-plate is made from a piece of sheet-brass and bent so as to form a pocket into which the arm will fit. This pocket piece is connected with the binding-post B. When the switch is thrown out the circuit is broken, unless a contact-point, C, is provided, from the under side of which a wire leads out to a second circuit. When the switch is in place, as shown in Fig. 16, the circuit is closed through A and B; but when the arm is thrown out the circuit through A and B is broken and that through A and C is closed.