In practice the measurements are generally made by means of the connection between electricity and magnetism. A current of electricity is a magnet. Whenever a current is flowing it is surrounded by a region in which magnetism can be felt. This region is called the magnetic field, and the strength of the field varies with the strength that is the number of amperes in the current. If a wire carrying a current be wound up into a coil it is evident that the magnetic field will be more intense than if the wire be straight, for it will be concentrated into a smaller area. Iron, with its peculiar magnetic properties, if placed in a magnetic field seems to draw the magnetic forces towards itself, and consequently, if the wire be wound round a core of iron, the magnetism due to the current will be largely concentrated at the ends of the core. But the main principle remains--in any given magnet the magnetic power exhibited will be in proportion to the current flowing. The switchboard at a generating station is always supplied with instruments called ammeters, an abbreviation of amperemeters, for the purpose of measuring the current passing out from the dynamos. Each of these consists of a coil of wire through which the current passes. In some there is a piece of iron near by, which is attracted more or less as the current varies, the iron being pulled back by a spring and its movement against the tension of the spring being indicated by a pointer on a dial. In others the coil itself is free to swing in the neighbourhood of a powerful steel magnet, the interaction between the electro-magnet, or coil, and the permanent magnet being such that they approach each other or recede from each other as the current varies. A pointer on a dial records the movements as before. In yet another kind the permanent magnet gives way to a second coil, the current passing through both in succession, the result being very much the same, the two coils attracting each other more or less according to the current. Another kind of ammeter known as a thermo-ammeter works on quite a different principle. It consists of a piece of fine platinum wire which is arranged as a "shunt"--that is to say, a certain small but definite proportion of the current to be measured passes through it. Now, being fine, the current has considerable difficulty in forcing its way through this wire and the energy so expended becomes turned into heat in the wire. It is indeed a mild form of what we see in the filament of an incandescent lamp, where the energy expended in forcing the current through makes the filament white-hot. The same principle is at work when we rub out a pencil mark with india-rubber, whereby the rubber becomes heated, as most of us have observed. The wire, then, is heated by the current passing through it, and accordingly expands, the amount of expansion forming an indication of the current passing. The elongation of the wire is made to turn a pointer.