Each car on a tramway system may thus be regarded as a bridge which completes an electrical circuit. When the driver moves his controller, current flows from the generating station at a high pressure, passes through the controller, operates the motors, and returns to the generating station at a low pressure. This typical circuit is completed through every car, so that the demand on the generating station at any moment is the sum of the demands of the cars at that moment. The business of the engineer at the generating station is to maintain the electrical pressure in the overhead wire at the normal level of 500 volts; and in order to do this on an ordinary tramway system it is found convenient to divide the overhead wire into half-mile sections, each of which has a separate main or 'feeder' from the generating station. The passenger can detect the change from one section to another by the click of the trolley wheel across the gap which insulates one half-mile section from another. At the same spot he can see the short square 'feeder-pillar' at the roadside (containing the switches by which current can be turned off from that section) and the cables which pass along the arm of the trolley standard and terminate in the overhead wire. On an extensive tramway system the power-supply arrangements become more complicated. The central generating station remains the primary source of power, but sub-stations are erected at convenient points between the central station and the outskirts of the tramway area. These sub-stations are secondary stations for the distribution of electricity. They receive power at extra-high pressure (5000 volts or more) from the central station; they contain special machinery for reducing the pressure to 500 volts for distribution to the various tramway feeders. The object of this arrangement is partly technical but mainly economical. Electric power can be transmitted at a lower cost in mains and with less loss of energy at high pressures than at low. Consequently when the termini of tramway routes are several miles from the generating centre, greater all-round efficiency is secured by transmitting current at high pressure to a number of well selected sub-stations. [Illustration: Fig. 4. Photograph of a car on a conduit section of the London County Council tramways. The centre line on the vacant track indicates the slot rail through which the 'plough' on the car passes to make contact with the conductors in the underground conduit. (Photograph reproduced by courtesy of Dick, Kerr and Company, Ltd.)] CHAPTER VI CONDUIT AND SURFACE-CONTACT TRAMWAY SYSTEMS