Where made: 
4.5 × 4.5 × 15.8 cm

This is a triode mercury vapour tube. Plate current is drawn when a positive voltage is applied to the grid. The grid then has no further control as ionisation of the mercury vapour maintains the, potentially rather high, current. Control is restored by switching off the thyratron, either by removing the anode voltage, or reducing the anode current to a very low value. The thyratron was an effective fast switch for high voltage circuits.