ignitrons

An ignitron is a type of gas-filled tube used as a high-power, high-voltage switch. It is a mercury-pool cathode device, meaning it uses a pool of liquid mercury as its primary source of electrons. The tube contains an anode, a control grid, and a small ignitor electrode, all enclosed in a vacuum-sealed container. The ignitor is crucial for initiating conduction. When a high-voltage pulse is applied to the ignitor, it momentarily vaporizes a small amount of mercury, creating a plasma arc that then allows the main current to flow between the anode and the mercury pool cathode.

Ignitrons were widely used in industrial applications requiring the switching of high currents, such as resistance welding, electric furnace control, and early high-power radar systems. Their ability to handle significant power made them suitable for these demanding roles. The control grid could be used to initiate the arc, but the ignitor was generally more reliable for starting conduction, especially at high currents.

Disadvantages of ignitrons included their bulkiness, the need for careful handling due to the mercury content, and limitations in switching speed compared to modern semiconductor devices. They also required a supply of cooling water to manage the heat generated during operation. While largely superseded by semiconductor switches like thyristors and IGBTs in many applications, ignitrons played a significant role in the development of power electronics and continue to be found in some specialized industrial equipment where their robust nature and high power handling capabilities are still valued.