XXIII.The primary standard of mutual inductance of the National Physical Laboratory
N F Astbury · The London Edinburgh and Dublin Philosophical Magazine and Journal of Science · 1938
The experience gained in recent years in the construction of standards of mutual inductance showed that it would be possible to obtain increased accuracy with the Campbell primary standard of mutual inductance by rewinding the secondary coil, so as to have a lower resistance, and to have a cross-section more nearly square. A new secondary coil possessing these features has been constructed. It has also been provided with auxiliary windings by means of which the mutual inductance may be varied continuously, without loss of accuracy, over a small range. This is achieved by having two additional windings, from one of which an integral number of turns may be selected, the other being in parallel with a small potentiometer, the potential circuit of which is put in series with the main secondary circuit. An interesting mechanical feature is that the adjusting and fixing screws for the new coil have been made from a synthetic resin.