{Contributed by H. Y. Margarv)

It is essential to become familiar with the units used in electrical measurement before the principles of wiring a building for lighting purposes can possibly be understood. The units used in electrical practice are based upon the fundamental units of length, mass, and time, and the C.G.S. (Centimetre, Gramme, Second) system of units is always employed.

A more vivid idea of the electrical units may be obtained by first gaining some knowledge of the following mechanical terms: -

Force is that which alters or tends to alter the state of rest or of uniform motion of a body.

The C.G.S. unit of force is the force which imparts a velocity of one centimetre per second to a mass of one gramme in one second. This unit is called a Dyne.

Work. An agent is said to do work when it causes the point of application of the force which it exerts to move through a certain distance.

A C.G.S. unit of work is done when a force of one dyne has been exerted through a distance of one centimetre. This unit is called an Erg.

Energy is the capacity of any agent for doing work. Thus a moving mass by virtue of its motion, or a compressed spring by virtue of the stress between its compressed particles, are each capable of doing work. The unit of energy is likewise the Erg:

Power is the rate at which work is done. This is expressed in ergs per second.