In order to maintain an arc discharge, a carbon rod or the cathode of a discharge
tube had to emit large quantities of electrons. This required a thermal emission from a
hot cathode. For carbon rod devices, contact ignition was the usual way to start the
A discharge tube, on the other hand, could be ignited by simply connecting it to a
serial ballast and a voltage source. Under influence of gamma- and cosmic radiation,
a spontaneous dark discharge could then develop into a glow discharge and finally
into an arc discharge. This process however, was quite unpredictable and could take
a long time if it happened at all. To overcome this uncertainty, the discharge had to
be forced to start more quickly and over time many methods to ignite a discharge
tube had been tested and applied, from tilt- and inductive ignition to glow- and
automatic ignition.