Calculation of a primary standard of mutual inductance of the Campbell type and comparison of it with the similar N. P. L. standard
David William Dye · Proceedings of the Royal Society of London Series A Containing Papers of a Mathematical and Physical Character · 1922
Abstract The standard mutual inductance devised and designed by Mr. A. Campbell and constructed in 1907-8 at the National Physical Laboratory has been one of the foundations of our alternating current measurements since that date. It will be sufficient here to note that the special feature in the design of the Campbell type of mutual inductance consists in a primary single-layer winding, so proportioned that the field due to it is practically zero over the region occupied by the secondary coil. By this means the dimensions of the secondary coil are rendered relatively unimportant, so that it may be an overwound many-layer winding, whereby a suitably large value of mutual inductance may be obtained.