The WL681/686 is an ignitron designed for the control of AC welding apparatus. The steel body has an evacuation pip at the base as well as the igniter electrode. The anode connection is via the twisted wire rope at the top. The working gas is mercury vapour produced from the reservoir of mercury sealed into the envelope.
The igniter and sealed pumping tube at the base.
Within the glass tube can be seen some of the mercury. When fully inverted the mercury fills the glass tube.
The wide metal tube envelope is 54 mm in diameter, and excluding the base pin is 210 mm tall.
Reference: Datasheet. Type WL681/686 was first introduced in 1946.