Strong currents of electricity are generated in the electric central stations and supplied to our homes, street lamps and so forth, in one of the two forms, either "alternating" or "direct." While many of us know which kind is furnished to our homes, everyone does not always understand the difference between the two.

The central station contains a number of powerful dynamo machines, driven usually by steam power. The positive and negative terminals of the dynamo are put in connection with the positive and negative main conductors which are to supply the district, and from these mains smaller conductors branch off to the houses or lamps. All these conductors are of copper, that metal when pure having seven times the conductivity of iron.

Different methods are in use for keeping the supply of electricity steady in spite of the varying demands made upon it. In some systems of distribution, instead of the two main conductors being one positive and the other negative, each is positive and negative alternately, the reversals taking place some hundreds of times per second. The currents are then said to be "alternating." When such reversals do not take place, the currents are said to be "direct."