Rectifier; Steel tank Mercury Arc Rectifier with ancilliary equipment, manufactured by British Thompson-Houston, circa 1923

Electricity is generated by power stations in the form of alternating current (AC). However, tube trains require direct current (DC) to operate. To manage this difference, mercury arc rectifiers such as this were used to convert the current from AC to DC. Installed at Old Oak Common sub-station this example supplied electricity to tube trains serving the Central Line between Ealing Broadway and Wood Lane stations. It is mounted on insulators, and would have been surrounded by safety barriers as the exterior steel tank was 'alive' with electricity when in use.