sub-stations become topographically central stations for a limited area. The objections to central stations mentioned above do not apply to sub-stations. They use neither coal nor water, they need not necessarily contain moving machinery, and if such machinery is erected in a sub-station it is of purely rotative type, such as electric motors and generators, which work without causing any noise or vibration; and finally the amount of space required is very small, so that the cost of land, even in the middle of the town, is no longer prohibitive. Where the supply is by alternating current no land at all is required. The converting apparatus consists mainly of transformers which may be put either under the pavement or in kiosks at street corners.
If a continuous current supply must be given to the householders, then there must be in the sub-station not only a conversion as to voltage, but also as to type of current. The conveyance of electric power from the central station to the sub-station is done most economically by means of three-phase current. This current is used to drive machinery which produces continuous current. Such an arrangement is shown diagrammatically in
