Mercury-Vapor Rectifier Tubes
High-vacuum diodes are limited to output currents of about 300mA maximum. Higher current outputs than this from vacuum diodes require tubes of excessive size if satisfactory efficiency is to be maintained. The mercury-vapor rectifier diode has a much higher current capacity because of the introduction of a small amount of mercury into its envelope. As the cathode is heated, a portion of the mercury vaporizes, filling the envelope with vapor. The mercury vapor greatly reduces the internal resistance of the diode, so that the tube can carry large values of current with only a small voltage drop.
The picture above shows the operation of the mercury-vapor rectifier and how the introduction of mercury vapor reduces the cathode-to-plate voltage drop. When the plate is positive with respect to the cathode, it attracts some of the electrons emitted by the cathode. Other electrons remain near the cathode, forming what is known as a space charge; this tends to limit the electron flow to the plate. The electrons traveling toward the plate encounter the mercury-gas atoms. When they collide, electrons are knocked loose from the mercury atoms. The mercury atoms that have thus lost electrons become positive ions. The positive ions neutralize some of the space-charge electrons which surround the heated cathode, so more electrons are able to move toward the plate. As they do, they strike additional ions. This process “snowballs,” producing a large flow of electrons between cathode and plate.
Remark: When mercury-vapour tubes are first placed in service, and each time after the mercury has been disturbed, such as by removal from the socket to the horizontal position, they should be run with filament voltage only for 30 minutes before applying high voltage. After that a delay of 30 seconds is recommended each time the filament is turned on.