This neutralising condenser is made of two parts. The top shaft is the turning spindle and inside the bottom casing (ebonite) there are two cylindrical brass plates. The inner plate is fixed and the outer plate rotates around it to alter the capacitance. .

Capacity is between 2 - 38pF over 6 rotations of the turning spindle.

A scan of the operating instructions can be downloaded from here. The instructions also contain details of the optional dial mechanism that attaches to the Neutrovernia and information on Gambrell coils.