Page:Motors and motor-driving (1902).djvu/311

Rh it, we know the resistance of the wire is one ohm. The analogy with the phenomena presented by liquids, such as water moving in pipes, is very close. Voltage corresponds to the pressure or head of water, ohmic resistance to the skin friction between the pipe and the running water, and amperage to the amount of water passing, say, in gallons per minute. Voltage, amperage, and ohmic resistance are measured in practice by special instruments called voltmeters, ammeters and ohmmeters.

The novice is sometimes troubled by having to familiarise himself with the notion that an electric current can flow through, or in, such a solid thing as a copper wire. Some people would perhaps find it easier to understand electric phenomena if conducting wires were made hollow. Persons who take this point of view may, however, console themselves with the reflection that a wire is not as solid as it looks, and also that some of the electric current runs along the outside. An electric current produces a magnetic condition, generally called a magnetic field, in the neighbourhood of a conductor—say a wire—along which it is passing. If a wire be twisted up into a helix, as at in fig. 1, and if an electric current be led into it through the flexible conductors or brushes  and 1, it would act in a feeble way like a magnet. If the current be stronger, it will act like a stronger magnet, and it will have one pole near where the current comes in and another pole where the current goes out. If a rod or core of soft iron c be slipped inside the coil of wire, and insulated or electrically separated from it, so as to prevent the electric current from passing through it instead of through the wire, the core will become a powerful magnet. As long as the electric current is passing it forms an electro-magnet. When the current ceases to flow, the iron will lose its magnetism—that is, provided it is soft. If a core of hard steel be used, it will take some time to become magnetized, but when it is magnetized it will be a permanent magnet. It is in this manner that soft iron and steel differ magnetically. If N, in fig. 1, be the north pole of a magnet