Editing Ignitron (section)

==Comparison with mercury-arc valve==
Although the basic principles of how the arc is formed, along with many aspects of construction, are very similar to other types of mercury-arc valves, ignitrons differ from other mercury-arc valves in that the arc is ignited each time a conduction cycle is started, and then extinguished when the current falls below a critical threshold.

In other types of mercury-arc valve, the arc is ignited just once when the valve is first energised, and thereafter remains permanently established, alternating between the main anode(s) and a low-power ''auxiliary anode'' or ''keep-alive circuit''.  Moreover, control grids are required in order to adjust the timing of the start of conduction.

The action of igniting the arc at a controlled time, each cycle, allows the ignitron to dispense with the auxiliary anode and control grids required by other mercury-arc valves.  However, a disadvantage is that the ignition electrode must be positioned very accurately, just barely touching the surface of the mercury pool, which means that ignitrons must be installed very accurately within a few degrees of an upright position.