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Middle of the Nineteenth Century

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which shows that the electric disturbance is propagated along the wire with the velocity c.^[1] 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,^[2] or conversely when units which have been defined electrodynamically^[3] 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;^[4] 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

↑ In referring to the original memoirs of Weber and Kirchhoff, it must be remembered that the quantity which in the present work is denoted by c, and which represents the velocity of light in free aether, was by these writers denoted by e/ ${\sqrt {2}}$ . Weber, in fact, denoted by c the relative velocity with which two charges must approach each other in order that the force between them, as calculated by his formula, should vanish.
It must also be remembered that those writers who accepted the hypothesis that currents consist of equal and opposite streams of vitreous and resinous electricity, were accustomed to write 2i to denote the current strength.
↑ i.e., defining unit electric charge as that which exerts unit ponderomotive force on a conductor at unit distance which carries an equal charge; and then defining unit current as that which conveys unit charge in unit time.
↑ i.e., defining unit current by means of the ponderomotive force which it exerts on an equal current, when the two currents low in circuits of specified form at a specified distance apart.
↑ Ann. d. Phys. xcix (1856), p. 10.

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[1] In referring to the original memoirs of Weber and Kirchhoff, it must be remembered that the quantity which in the present work is denoted by c, and which represents the velocity of light in free aether, was by these writers denoted by e/ ${\sqrt {2}}$ . Weber, in fact, denoted by c the relative velocity with which two charges must approach each other in order that the force between them, as calculated by his formula, should vanish.
It must also be remembered that those writers who accepted the hypothesis that currents consist of equal and opposite streams of vitreous and resinous electricity, were accustomed to write 2i to denote the current strength.

[2] .e., defining unit electric charge as that which exerts unit ponderomotive force on a conductor at unit distance which carries an equal charge; and then defining unit current as that which conveys unit charge in unit time.

[3] .e., defining unit current by means of the ponderomotive force which it exerts on an equal current, when the two currents low in circuits of specified form at a specified distance apart.

[4] Ann. d. Phys. xcix (1856), p. 10.

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