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.