The unipivot galvanometer was invented by Robert W. Paul in 1903. It is a classic of careful instrument design and continued in production for over 50 years.
As in the more common form of moving coil galvanometer (D'Arsonval) a coil is pivotted between the poles of a permanent magnet and experiences a torque when a current flows.
However, the coil here is circular and cleverly pivoted on a single spike (the unipivot) at its centre of gravity, with the result that the device is not disturbed by accelerations and could be used to make measurements in moving vehicles.
The design was further refined by an automatic lock that protected the delicate mechanism from damage when not in use. Paul staff are reputed to have played football with an instrument in its leather case to demonstrate its ruggedness.
This example was made in 1940. Similar examples are found with Department of Defence markings from WWII.
another example from 1938 (back)
another example from 1938 (front)
