, as in [subarticle 6.1]. It will be necessary to explain a few small points of nomenclature mid notation.
A vector is a directed physical quantity; for example, the electric force at
point is a vector. This example also shows that we have to conceive vectors which have analogous significations at different points of space. Such a vector is the electric force which may have a distinct magnitude and direction at each point of space, but expresses at all points one definite physical fact. Such a vector will be a function of its position, that is to say, of the coordinates of the point (
) of which it is that characteristic vector.
Let (
) be any such vector. Then
