Campbell standard of mutual inductance. Mr A. Campbell designed this standard of mutual inductance in 1907-8. Two single-layer copper coils (upper and lower centre) are wound onto the outside of a marble cylinder containing a multiple-layered inner copper coil (not seen). The current passing through the inner coil induces a specific voltage in the outer coils. This phenomenon is known as mutual inductance. The Campbell standard of mutual induction was used to determine the Ohm (a unit of electrical resistance). Photographed in 1960.