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Special types of single-phase mercury-arc rectifiers are the Ignitron and the Excitron. The Excitron is similar to other types of valve described above but depends critically on the existence of an excitation anode to maintain an arc discharge during the half-cycle when the valve is not conducting current. The Ignitron dispenses with excitation anodes by igniting the arc each time conduction is required to start. In this way, ignitrons also avoid the need for control grids.
In 1919 the book "Cyclopedia of Telephony & Telegraphy Vol. 1" described an amplifier for telephone signals that used a magnetic field to modulate an arc in a mercury rectifier tube. This was never commercially important.
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