On a standard of mutual inductance

1.Introductory .—In many electrical measurements, such as those of capacity and inductance, as well as in the magnetic testing of iron, an accurately known standard of mutual inductance is of great value. It is sometimes convenient to derive such a standard from the standard unit of resistance, and this may be done in several ways, for example, by the well-known method of the ballistic galvanometer; or by Carey Foster’s method the mutual inductions may be tested against a condenser whose capacity has been found in terms of resistance and frequency by Maxwell’s commutator method; or it may be obtained directly in similar terms by the help of an unknown inductance by the Hughes-Rayleigh method. In the National Physical Laboratory I have used both of these latter methods (with the help of a vibration galvanometer) to obtain a working standard of mutual inductance. But this procedure is somewhat illogical, seeing that the unit of resistance has been itself commonly determined by the aid of mutual inductances calculated from the dimension of the coils or other conductors used; thus for the highest accuracy it is desirable to revert to a standard whose value can be determined solely from the geometrical dimensions. Accordingly, some eighteen months ago, I took in hand the investigation of a suitable design for such a standard, and I proceed to describe the result at which I arrived.