From Virtual Valve Museum:
The cathode, anode and auxiliary electrode consist of mercury pools. It uses a capacitive igniter. The ignition electrode extending down into the mercury pool and is insulated from it by a thin coating of dialectric material. A high voltage applied to the ignition electrode causes a small amount of electron emission. At this stage a high voltage momentarily applied to the auxiliary electrode causes ionisation of the mercury vapour which then starts the main discharge.