The mercury-arc rectifier and supply to electric railways

The introduction in the 1890s of the heavy-duty LVDC rapid-transit railway, drawing current from an AC power station, required some form of rectifier for converting the supply to DC in the conductor rail or contact wire. The development of the HVDC railway by 1915 likewise necessitated rectifiers. Though the principle of using the mercury arc to convert AC to DC was well understood by 1905, the first valves, owing much to Cooper-Hewitt, were too low in capacity to serve railway needs before the early 1920s, and railway electrification relied on rotary converter sets. By the 1920s, both the glass-bulb and steel-tank types of mercury-vapour rectifier were sufficiently developed to be used in railway service, first for battery charging, then as supplements to rotary sets, and finally as main power rectifiers. Throughout the 1930s, both the glass-bulb type, and the multianode steel-tank rectifier were installed in automatic substations, remotely controlled. In Great Britain, London Transport pioneered their extensive use. Between 1945 and 1965, the air-cooled, pumpless rectifier found favour, but the developments in solid-state semiconductor rectifiers led to their replacement throughout industry. The types of mercury-arc rectifier mounted on locomotives proved unreliable in this service, and hence relatively few were used in this capacity. In stationary use, all major types of mercury-arc rectifier performed well, and in railway service the majority were employed in lineside substations