This device is a mutual inductor designed for the calibration of a ballistic galvanometer. The primary coil carries a fixed current and the secondary coil can be flipped through 180º to produce a calibrated flux linkage. The motion is driven by a spring released by a catch on the top plate. The tall box contains two pairs of coils mounted above one another with opposite windings to eliminate the effects of the Earth’s or other external fields.

The original NPL calibration stated that with a primary current of 10 A the flux linkage was 1,017,000 line turns. In modern units this is equivalent to 0.01017 Wb. In 1957 the NPL gave 1,021,000 line turns.

Inscriptions / Translations:  INDUCTOR
THE CAMBRIDGE SCIENTIFIC INSTRUMENT Co. Ltd.
CAMBRIDGE ENGLAND No. 4098
No. 2983