Henderson Firing Gear
Henderson Firing Gear, often referred to by different variations, was a gyroscopic mechanism attached to a British gun director which stabilised the director layer's telescope and was wired in series with the firing circuit to convert his firing pistol from a direct firing pistol to a permissive firing key that would discharge the broadside when the ship next rolled through a reference plane. In effect, it made firing on the roll an automatic affair.
Impetus and Design
By at least mid 1917, the equipment was being fitted to director installations for main armaments in all ships fitted or being fitted with directors. A C.O.S. was fitted in case the device should require a duplicate alternator supply and in the G.D.T., a special switch allowed main and auxiliary firing circuits to function with or without the Henderson equipment.
In 1917, it was approved that all installations should be altered so that both main and auxiliary firing circuits were energised by the gear by use of a double contact system. This required replacing the existing night sight and director/Henderson C.O.S.es with two new ones. 
- Annual Report of the Torpedo School, 1916. Plate 87.
- Annual Report of the Torpedo School, 1917. p. 230.