1. THE FORMULA. A well-constructed standard of self-inductance may be measured with considerable precision in terms of a resistance. If its value be computed from its dimensions, the two results should agree, provided the resistance is known in absolute measure. This affords a method of determining resistance in absolute measure, the success of which depends, of course, on how accurately the self-inductance can be computed, as well as upon the accuracy of the measurements. Kirchhoff's method of determining resistance in absolute measure consists essentially in finding by experiment the mutual inductance of a pair of coils in terms of a certain resistance, the value of the mutual inductance having been calculated from the dimensions of the coils and their distance apart. Placing these two values equal to each other, the value of the given resistance becomes known in absolute measure. The mutual inductance of such a pair of coils can be determined experimentally by two measurements of selfinductance, in one of which the current flows in the same direction in the two coils and in the other it flows in opposite directions. If L and L'are the two values of self-inductance determined experimentally, L, and L, the self-inductances of the two coils separately, and M their mutual inductance, then