Control panel indicator; Manor House sub-station control panel from Wood Green control room, 1938

This control panel was used to remotely operate the supply of electricity from Manor House sub-station on the Piccadilly line. The DC system used on the Underground is quite simple, but it requires thick cables and short distances between sub-stations because of the high currents involved. By linking the operation of several sub-stations to a central control room, one person could manage the distribution of electricity over a greater area. London Underground is one of the few networks that uses a four-rail system - the third rail beside the track carries +420 Volts DC, while the central rail carries -210 Volts DC. This system was developed to avoid the problem of the return current travelling through the iron tunnel rings or nearby water pipes rather than through the earthed running rails.