30. A switch is an appliance interposed in an electric circuit for the purpose of readily, and without danger, opening or closing that circuit. For large currents some form of knife switch, such as that shown in Fig. 20, is employed. The two copper levers l and l1 carry plates of copper in the shape of knife blades, shown at k in the side view. These knife blades fit between elastic metallic plates shown in perspective at r. There are four pairs of these plates connected respectively to the metallic pieces a, b, c, and d, so that each knife blade connects with two pairs. The levers are pivoted at p and p1, and are moved by the handle h.

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Fig. 20.

The line wires are both cut, and the ends of one are connected respectively to a and b, while the ends of the other are connected to c and d. It will now be seen that when handle h is pushed down, the circuit is complete, but when it is pulled up, the circuit is broken in both wires. The base is of slate, and is shown at m.

31. Plug switches are frequently used for the connection between the house wiring and the street mains. This switch consists simply of a conical metal plug p, Fig. 21, with a wooden knob k for a handle, and is pressed into contact with two blocks b, b', forming part of the conductor circuit, and mounted on an insulating base i secured to the wall.

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Fig. 21.

32. A form of double-pole switch for currents up to 50 amperes is shown in Fig. 22. The positive and negative leads are brought up to the hole in the base b, and connected one to each of the terminals shown, by means of the screws, s, s'. The leads for the lamps are connected in a similar manner to corresponding terminals on the other side of the switch, and the circuit is completed by forcing the arm a into contact with these terminals, thereby bridging over the gap between them. The rubber knob k is fastened to a pin passing vertically through the frame f and secured to the springs c, c' at the lower end in such a way as to form a togglejoint. When the knob is drawn upwards, the springs are compressed, and, on passing the center, they suddenly force the contact arm downwards. In like manner, on pressing the knob down, contact is again broken.

33. Fig. 23 shows a double-pole switch for small currents. It is usually fastened to the wall, the screws passing through the base. The cylinder e, made of china or other insulating substance, has brass contact plates p on op-posite sides, against which press brass or copper springs when the cylinder is in the position indicated in the illustration. Four ter-minals are provided, lettered a, b, c, d, and the wires for connection to them are brought up through the holes in the base, one of which is visible. The incoming wires, positive and negative, are connected to the terminals b and d, and the outgoing wires to a and c. The springs a', b', c', d' are riveted to the terminals a, b, c, d, respectively, so that when the switch is turned to the position shown, the circuit is completed between terminals a and b, and between c and d. A quarter-turn breaks the contact, for the springs then rest only on the china cylinder. A screw cover is provided to enclose the body of the switch, the handle alone projecting.

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Fig. 22.

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Fig. 23.

34. A single-pole switch may only be used for a few lamps, as, for instance, an electrolier, or the lights in a small room. In Fig. 24 the lead is brought to the screw terminal marked a, and the wire from the lamps is secured to b, the return current passing through the other lead, which is connected directly to the main. When the key k is turned through 90°, the spring s is pressed against the tongue t, completing the circuit between a and b. Another quarter-turn releases the spring and extinguishes the lights. The wires are brought to the contacts through a hole h in the base, and a cover is fitted over, as in the case of the last mentioned switch.

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Fig. 24.

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Fig. 25.

35. Flush switches are so called because they are intended to be let into the wall, hardly projecting beyond the surface. The mechanism is usually of the same style as that in Fig. 23 or Fig. 24, but the cover is simply a plate, and the body of the switch is concealed. Another form is given in Fig. 25, called a Flush Double Push Switch. This is a single-pole switch, and is operated by pressing one or the other button, one being out when the other is in. The contact lever l is forced between the brass leaves c, c' by means of a spring and toggle-joint, somewhat as in Fig. 22. One wire is wound around the screw s, and is held under the washer, and the other wire is secured in a similar manner on the other side. These two ends of wire belong to the same lead, which is cut at the point where the switch is to be inserted. The interior of the switch is protected by plates which are fitted over the sides.

The external appearance of the usual form of flush switch is simply a nickeled brass plate on a wooden block let into the wall, a recess being provided in the plate for the key, which, therefore, projects only a slight distance from the level of the plate.

36. When connecting up switches, great care must be taken to enter the wires at the correct terminals, and the path of the current should be traced through, in order to be sure that the closing of the switch will not short circuit the system.