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Double throw switches. made in the circuit, one between S and a and one between S and b.

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Figures 99-100. Double throw switch. Diagram Fig. 93 shows a double-throw, singlebreak switch and diagram Fig. 94 is a double-throw, double-break switch. In these two figures it will be seen that if S is moved in the direction of arrow e, wire P will be connected with wire N; and if it is moved in the direction of arrow f, P will be connected with N1; hence there are two directions in which the switch can be moved to close the circuit.

Diagram Fig 95 shows a multi-throw, single-break switch, and diagram Fig. 96 is a multi-throw, doublebreak switch. In the first figure the wire P can be connected with any one of the six wires leading off from contacts b, c, d, e, f, g, by the movement of S, but in each case the circuit is only opened at one point.

In Fig. 96, however, wire P connects with all the contacts of the inner circle, these being joined by the ring R, and S is not directly connected with P, hence when it is moved so as to connect any two contacts it closes the circuit at two points, just as in the case of Fig. 92.

The switches so far explained are of the single pole type—that is, they open or close the circuit at one place only. The wire P, in these diagrams may be considered as one of the leads coming from the generator.

Single and double throw switches.

In Fig. 97 to 100 switches of the two pole type are shown.

The switches used on switchboards are almost invariably of the type known as knife switches. They are given this name from the fact that the blade moves in a plane perpendicular to the switchboards surface, like a meat shaver. Switches in which the blade moves in a plane parallel with the surface of the board are occasionally employed, when it is desired to connect one terminal

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Figure 104 Figures 101-104. Different types of switches. with any one of three or more other terminals. Switches of this latter type are designated as side-throw switches. The appearance of a simple knife switch, seen from the side, is shown in Fig. 101, in which S represents the switch blade, A and N the terminals, M the switchboard surface, and f g the studs by means of which the terminals are secured in place and are connected with the conductor bars, back of the board.

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Large switches.

The switch Fig. 101 is called a single-throw switch, because the blade can only be thrown in one direction to close the circuit. The design shown in Fig. 102 is called a double throw switch from the fact that the blade will close the circuit if thrown in either direction. If the switch has but one blade it can only open or close the

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circuit in one wire, and is, therefore, called a single-pole switch; but if S consists of two blades, as shown in Fig 103, which is a plan of Fig. 102, then if the two blades are insulated from each other, each can open or close the

Quick break switches.

circuit through one wire. Switches of this latter type are called double-pole, and if S consists of three insulated blades the switch is three-pole.

In Fig. 102, as well as in Fig. 101, the current passes through A, therefore there must be a perfect fit between A and S. In Fig. 104 the current does not enter through A, hence a loose fit will answer as well as a tight one. In this last figure the current passes through the blade S

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Figure 114. from N to N; therefore at these points a good fit is required, and N must exert a considerable pressure against S. It can be readily seen that if the blade S is raised in Fig. 104 it will produce two breaks in the circuit. On

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