Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/279

 which shows that the electric disturbance is propagated along the wire with the velocity c. Kirchhoff's procedure has, in fact, involved the calculation of the capacity and self-induction of the wire, and is thus able to supply the definite values of the quantities which were left undetermined in the general equation of telegraphy.

The velocity c, whose importance was thus demonstrated, has already been noticed in connexion with Weber's law of force; it is a factor of proportionality, which must be introduced when electrodynamic phenomena are described in terms of units which have been defined electrostatically, or conversely when units which have been defined electrodynamically are used in the description of electrostatic phenomena. That the factor which is introduced on such occasions must be of the dimensions (length/time), may be easily seen: for the electrostatic repulsion between electric charges is a quantity of the same kind as the electrodynamic repulsion between two definite lengths of wire, carrying currents which may be specified by the amount of charge which travels past any point in unit time.

Shortly before the publication of Kirchhoff's memoir, the value of c had been determined by Weber and Kohlrausch; their determination rested on a comparison of the measures of the charge of a Leyden jar, as obtained by a method depending on electrostatic attraction, and by a method depending on the