The nature of the current generated at a power station is determined to a great extent by the size of the district to be supplied. Generally speaking, where the current is not to be transmitted beyond a radius of about two miles from the station, continuous current is generated; while alternating current is employed for the supply of larger areas. In some cases both kinds of current are generated at one station. [Illustration: PLATE V. _By permission of_ _C. A. Parsons & Co._ LOTS ROAD ELECTRIC POWER STATION, CHELSEA.] If continuous current is to be used, it is generated usually at a pressure of from 400 to 500 volts, the average being about 440 volts; and the supply is generally on what is known as the three-wire system. Three separate wires are employed. The two outer wires are connected respectively to the positive and the negative bus bars running along the main switchboard, these bars receiving positive or negative current directly from the dynamos. The outer wires therefore carry current at the full voltage of the system. Between them is a third and smaller wire, connected to a third bar, much smaller than the outer bars, and known as the mid-wire bar. This bar is not connected to the dynamos, but to earth, by means of a large plate of copper sunk into the ground. Connexion between the mid-wire bar and the outer bars is made by two machines called “balancers,” one connecting the mid-wire bar and the positive bus bar, and the other the mid-wire bar and the negative bus bar. If the pressure between the outer bars is 440 volts, then the pressure between the mid-wire bar and either of the outer bars will be 220 volts, that is just half.