We have seen that electro-motive force is measured in volts, and that the definition of a volt is that electro-motive force which will cause a current of one ampere to flow through a conductor having a resistance of one ohm. If we make a galvanometer with a long coil of very thin wire having a high resistance, the amount of current that will flow through it will be proportionate to the electro-motive force. Such a galvanometer, fitted with a carefully graduated scale, in this way will indicate the number of volts, and it is called a _voltmeter_. If we have a galvanometer with a short coil of very thick wire, the resistance put in the way of the current is so small that it may be left out of account, and by means of a graduated scale the number of amperes may be shown; such an instrument being called an _amperemeter_, or _ammeter_. For making exact measurements of electric currents the instruments just described are not suitable, as they are not sufficiently accurate; but their working shows the principle upon which currents are measured. The actual instruments used in electrical engineering and in scientific work are unfortunately too complicated to be described here. CHAPTER VIII THE INDUCTION COIL The voltaic cell and the accumulator provide us with currents of electricity of considerable volume, but at low pressure or voltage. For many purposes, however, we require a comparatively small amount of current at very high pressure, and in such cases we use an apparatus called the _induction coil_. Just as an electrified body and a magnet will induce electrification and magnetism respectively, so a current of electricity will induce another current; and an induction coil is simply an arrangement by which a current in one coil of wire is made to induce a current in another coil. Suppose we have two coils of wire placed close together, one connected to a battery of voltaic cells, with some arrangement for starting and stopping the current suddenly, and the other to a galvanometer. As soon as we send the current through the first coil, the needle of the galvanometer moves, showing that there is a current flowing through the second coil; but the needle quickly comes back to its original position, showing that this current was only momentary. So long as we keep the current flowing through the first coil the galvanometer shows no further movement, but as soon as we stop the current the needle again shows by its movements that another momentary current has been produced in the second coil. This experiment shows us that a current induces another current only at the instant it is started or stopped, or, as we say, at the instant of making or breaking the circuit.