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278 implication we do not know precisely what an electric current is, but we have all requisite knowledge about it for practical purposes. An electric current is a something which occurs in a conductor, i.e. a piece of metal when it connects two points between which there is electric pressure. The current may be only momentary, as in the case of Franklin's kite with a wire attached which was sent up into a thundercloud, or when a so-called Leyden jar is discharged; or it may last a little longer, as when we discharge a dry cell. Finally it may last some hours, as in the discharge of an accumulator; but while it lasts its characteristics—the effects it produces—are the same. It heats the metal through which it flows, and it produces magnetism in the neighbourhood. The difference of electric pressure between two points is termed potential difference, and it is measured in 'volts.'

Every conductor opposes a certain amount of resistance to the passage of the electric current. This resistance is measured in what are called 'ohms,' the ohm being a unit of electrical resistance. It is the electrical resistance of a rod of copper of a certain length and thickness when at a certain temperature. The electric pressure or difference of potential which will send a certain amount of current, called an 'ampere,' through a conductor the resistance of which is an ohm, is one volt. It is half the pressure approximately existing between the terminals of an ordinary accumulator, and is about enough to heat half an inch of thin platinum wire red-hot. Volts, amperes, and ohms are all mutually dependent units of measurement—thus, if along any wire, the resistance of which we know to be an ohm, there is flowing one ampere of current, we know that the electric pressure or voltage between the ends of that wire is one volt. If there be a difference of pressure at the ends of the conductor of one volt, and we find that there is half an ampere of current flowing, we know that the resistance is two ohms. If we have a pressure of two volts maintained between two points, and we connect those two points by a wire, and find that two amperes of current flow through