is an invariant. When we put this implies that a certain quadratic form is an invariant.
Let us suppose that the constitutive relations connecting with are given by the circumstance that the invariant reciprocal to
is an invariant multiple of
This assumption preserves the analogy with the electron equations where the two fundamental integral invariants of the second order are reciprocals with regard to the quadratic form
In the present case the relations between the two sets of vectors are of the type
(2)
where Δ denotes the determinant
To obtain the other constitutive relations we start with the assumption that there is an integral invariant of the type[1]
↑This assumption is justified by the remark made on p. 249.