Mercury Arc Rectifier
The operation of the mercury are rectifier depends upon the fact that a tube containing mercury vapor under a low pressure and provided with two electrodes, one of mercury and the other of some other conductor, offers a very high resistance to a current tending to pass through the tube from the mercury electrode to the other electrode, but offers a very low resistance to a current tending to pass through the tube in the opposite direction. Current passes from the metallic electrode to the mercury electrode through an are of mercury vapor which is established in the tube by tilting it so the mercury bridges the gap between the mercury and an auxiliary electrode just for an instant.
The absence of moving parts to got out of order is an advantage possessed by this rectifier over the motor-generator. The charging current from the rectifier cannot, however, be reduced to as low a value as with the motor-generator, and this is a disadvantage. This rectifier is therefore more suitable for larger shops, especially where electric truck and pleasure cars are charged.