What are the ignitrons and excitrons that the early electric locomotives used?
These are different kinds of rectifying devices used to convert alternating current to direct current. In the early days, this could be done only with vacuum-tube technology as semiconductor technology could not handle the high voltages and currents that need to be switched in an application such as in a locomotive. An excitron is a mercury vapour rectifier which uses a pool of liquid mercury as its cathode. An arc is maintained between this cathode and an auxiliary excitation anode, which maintains a concentration of ionized mercury in the tube. Current flows between the main anode and the cathode by mercury ions only when the main anode is positive with respect to the cathode. An ignitron is also a mercury vapour rectifier. However, in this an arc is not maintained continuously between the anode and the cathode. The arc exists while the anode is positive but is extinguished as the polarity switches. Instead, an igniter electrode of a semiconductor material such as silicon carbide is
