Discussion on “an imperfection in the usual statement of the fundamental law of electromagnetic induction”

Chas. P. Steinmetz (by letter): Mr. Hering's paper is interesting in that it draws attention to a looseness in the form of expressing this law, which is frequently the cause of serious misunderstanding, and the waste of much energy and time. For instance, in the attempts to invent a coil-wound unipolar machine much useless effort could have been avoided by a clearer distinction between the general law and the special case of its application to a continuous closed conductor. While Mr. Hering's experiment is interesting in showing an instance of a closed electric circuit in which the number of interlinkages with the lines of force changes without inducing an electromotive force, it is not startling to me, as the reverse case, the electromagnetic induction of an electromotive force, in a closed circuit, without any change of the number of interlinkages of the circuit with the magnetic flux, is illustrated by practically every unipolar machine. There are thousands of kilowatts of such machines now in commercial operation.