Middle of the Nineteenth Century. 259 



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 ,f or conversely when units 

 which have been defined electrodynamicallyj 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 re- 

 pulsion 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 



* In referring to the original memoirs of Weber and Kirchhoff, it must he 

 remembered that the quantity which in the present work is denoted by e, and 

 which represents the velocity of light in free aether, was by these writers denoted 

 by c/V'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 2t to denote the current-strength. 



f 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 flow in circuits of specified 

 form at a specified distance apart. 



Ann. d. Phys. xcix (1856), p. 10. 



S 2 



