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There are many people whose acquaintance with electricity consists mainly in paying the electric light bill. To such the instruments whereby electricity is measured will make a specially interesting appeal.

Current is sold in Great Britain at so much per Board of Trade Unit. To state what that is needs a preliminary explanation of the other units employed in connection with electric currents.

The public electricity supply in any district is announced to be so many volts, it may be 100, 200 or perhaps 230, but whatever it be, it is always so many "volts." Then the electrician speaks lightly of numbers of "amperes," he may even talk of the number of "watts" used by the lamps, while occasionally the word "ohm" will leak out. Among these terms the general reader is apt to become completely fog-bound. But really they are quite simple if once understood, and, as we shall see in a moment, there are some very remarkable instruments for measuring them, some of which exhibit a delicacy truly astonishing.

It is well at the outset to try and divest ourselves of the idea that there is anything mysterious or occult about electricity. It is quite true that there are many things about it very little understood even by the most learned, but for ordinary practical purposes it may be regarded as a fluid, which flows along a wire just as water flows along a pipe. The wire is, electrically speaking, a "hole" through the air or other non-conducting substance with which it is surrounded. A water-pipe being a hole through a bar of iron, so the copper core of an electrical wire is, so far as the current is concerned, but a hole through the centre of a tube of silk, cotton, rubber or whatever it be. Electricity can flow through certain solids just as water can flow through empty space.

Water will not flow through a pipe unless a pressure be applied to it somewhere. In a pipe the ends of which are at the same level water will lie inert and motionless. Lower one end, however, and the pressure produced by gravity—in other words, the weight of the water—will cause it to move. In like manner pressure produced by the action of a pump will make water flow. On the other hand, when it moves it encounters resistance, through the water rubbing against the walls of the pipe.

Similarly, an electrical pressure is necessary before a current of electricity will flow. And every conductor offers more or less resistance to the flow of current, thus opposing the action of the pressure. Before current will flow through your domestic glow-lamps and cause them to give light there must be a pressure at work, and that pressure is described as so many volts.

A battery is really a little automatic electrical pump for producing an electrical pressure. And the volt, which is a legal measure, just as much as a pound or a yard, is a certain fraction of the pressure produced by a certain battery known as Clark's Cell. It is not necessary here to say exactly what that fraction is, but it will give a general idea to state that the ordinary Leclanche or dry cell, such as is used for electric bells, produces a pressure of about one and a half volts.

Thus we see the volt is the electrical counterpart of the term "pound per square inch" which is used in the case of water pressure.

A flow of water is measured in gallons per minute. An electrical current is measured in coulombs per second. Thus the coulomb is the electrical counterpart of the gallon. But in this particular we differ slightly in our methods of talking of water and electricity. Gallons per minute or per hour is the invariable term in the former case, but in the latter we do not speak of coulombs per second, although that is what we mean, for we have a special name for one coulomb per second, and that same is ampere. One ampere is one coulomb per second, two amperes are two coulombs per second, and so on.