It has a low-pressure gas filling, whose composition is quite hard to find out - maybe a production secret? After Ratheiser's "Tube Bible" it contains Mercury vapour, somewhere else I read about mixtures of mercury and some inert gases - judging from the intense blueish glow (different to pure Mercury) of the arc I assume a gas mixture between Mercury and Argon, Crypton or Xenon. As with all gas-filled rectifiers, it has to be preheated (about 15 s). System voltage drop typically is about 7 V.

Electrode assembly is really primitive: the two plates - simple flat rectangular pieces of tin - are placed closely at either side of the cathode filament.

For some additional information and safety hints on gas-filled rectifiers see page 83.htm.