[Contributed by H. Y. Margary)

As in this and in the subsequent chapters a number of technical terms will be employed, such terms will be here fully explained save where they apply to fittings, when a general explanation will be given in this chapter, while a detailed description of the various forms and patterns will be given in the chapter upon "Accessories."

A Circuit means a system of conductors which will enable a current of electricity to pass through it.

A Switch is a contrivance for completing or breaking an electric circuit, and it is by means of a switch that a current may be allowed to flow or prevented from flowing in a circuit at will. The act of completing or "making" a circuit is called "switching on," and that of breaking a circuit is called "switching off."

A Fuse or cut-out is a contrivance for automatically cutting off a current from a circuit should too much current flow through it. The conductors in a building are of such a size that they can carry sufficient current to light the lamps with which they are connected. Should more current flow through the conductors than the amount they are intended to carry they become heated, thus destroying the insulating material and setting fire to any adjacent inflammable material. A fuse or "cut-out," which is composed of a piece of easily fusible wire, prevents this danger by melting when a current passes through it larger than it is intended to take.

A Short Circuit

Any circumstance which allows the current passing through a conductor to increase without limit is said to short circuit the conductor. A short circuit occurs when metallic contact is made between the terminals of a lamp holder, or when, through faulty insulation of cables, electricity escapes to earth.

A Meter is a contrivance for measuring the amount of electrical power consumed by a number of lamps.

A Distributing Board is a contrivance for enabling die wiring of a building to be split up into a number of separate circuits.

If all the lamps in a large building are connected directly to the two supply mains, and should a short circuit occur, a very large amount of electrical energy will pass through the mains and probably cause a fire. To prevent short circuits being formed in buildings carrying very large currents, most supply companies have a rule that no wiring system in which there is a circuit carrying more than ten amperes will be connected with their mains. For this reason boards conveniently arranged for splitting the wiring into separate circuits are used, each circuit being protected by a separate fuse.

In Fig. 140 distributing boards are illustrated diagrammatically. They consist essentially of a slab of some incombustible material to which two strips of copper or brass, A, A, called "omnibus bars," or more commonly "bus bars," are attached. At the ends of these bus bars are binding screws B, B, for the connection of the house service mains. Connections are also made between the bus bars and smaller binding screws C,C, to which the separate circuits are connected. The main distributing board usually has separate switches for each circuit.