_The Switches_ shown in the middle of the board, are enlarged in Fig. 232, and are used for closing the connections with the generators and lines running to various parts of the field to be lighted or furnished with power.
The switch handles are made usually of wood or hard rubber; the blades are of copper. The connections are soldered into the sockets shown upon the ends of the screws which project beyond the back of the switch-board.
The upper row of figures as shown in Fig. 230 and enlarged in the engraving, 231, are _circuit-breakers_. The use of these is analogous to that of the safety-valve upon a steam boiler, so that when the pressure in the circuit exceeds that at which it is set the “breaker” opens the circuit and thus prevents damage.
In this case, the main contact is formed by means of a laminated brush while the final stroke is made on carbon, the motion of this breaker is by means of a toggle-joint which so multiplies the power applied that it does not require much of an effort to close it; this device maintains the same speed in operating the breakers when the circuit-breaker is tripped.
_A Rheostat_ is a device for controlling the amount of electricity in a conductor—by the insertion of coils of wire in a box—which may be successively switched in or out of the main circuit by means of a lever and button-switch. The best place to install a rheostat is on a wall or post, as the resistance transforms a portion of the electric energy into heat, which heat must be dispersed into the atmosphere.
_A transformer_ is an induction coil employed usually for lowering electric pressure, but it may also be used for raising the same, in which case it is sometimes called a _booster_. _A compensator_ is a transformer which works automatically.
_Ammeters_ record _the quantity of current flowing through the circuit, in amperes_. _Voltmeters_ record _the pressure_ or strength of the current in _volts_.
An _Ampere_ is an electric current which would pass through a circuit whose resistance is one ohm under an electro-motive force of one volt. A _Volt_ is an electro-motive force of sufficient strength to cause _a current of one ampere to flow against a resistance of one ohm_.