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. [Illustration: FIG. 13 FIG. 14 FIG. 15 FIG. 16 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. Binding-posts and Connectors To make quick connections between wires and other parts of electrical apparatus, binding-posts are the most convenient device, since the turn of a screw binds or releases a wire instantly. Binding-posts may be made in many forms, but the simple ones that a boy will need can be made from screw-eyes, burrs, stove-bolts, and nuts, together with thin strips of metal and nails. Five simple posts are shown in Fig. 17. A is made from a screw and two burrs, B from a screw-eye and two burrs, and C from a thin plate of metal and two screws, with oval or round heads. This last, however is more of a connector than a binding-post. The ends of the wires to be connected should be caught under the screw-heads or between the burrs before the screws are driven down.