80 Prof. B. Clausius on the Connexion between 



upon other electric currents or other magnets the same forces 

 as the given magnet. Magnetism can thus, according to 

 Ampere, be accounted for, without assuming the existence of 

 a second agent besides electricity, from the existence of those 

 electric currents. 



According to this explanation, by the term Magnetism 

 we are to understand only an electrodynamic conception ; 

 and hence, in every system of measures, the unit which is to 

 be employed for measuring magnetism must be chosen in ac- 

 cordance with that conception — that is, so that the product of 

 the unit of magnetism and the unit of length, and the product 

 of unit current and unit area, become equivalent quantities. 

 Therefore, if [m] denotes the unit of magnetism, and \_i] the 

 unit of current- intensity, and, further, [L] the unit of length 

 and accordingly [L 2 ] the unit of area, the following equation 

 must hold : — 



[ OT L] = [iL 2 ]; (1) 



from which follows 



H = [»L] (2) 



Now, since the unit of current-intensity is the intensity of a 

 current in which, in the unit of time, the unit of electricity 

 passes through a cross section, we have, if [g] denotes the unit 

 of electricity and [T] the time-unit, to put 



by which the preceding equation is changed into 



[m]-[«LT-'] (3) 



This equation expresses the relation between the units of 

 magnetism and electricity corresponding to Ampere's expla- 

 nation of magnetism. If we apply it to the electrostatic 

 system of measures, and hence define the quantity [«], which 

 in this case must, in order to distinguish it from the electro- 

 dynamic unit of electricity, be written [0 J , by the equation 



M-ptfLlT- 1 -], (4) 



in which [M] denotes the unit of mass, we get for the defini- 

 tion of the electrostatic unit of magnetism the following equa- 

 tion : — 



[m s ] = [MlUT- 2 ] (5) 



It was this equation that gave rise to the above-mentioned 

 objections, because it differs from Maxwell's, which reads as 

 follows : — 



[>J = [M*L*] (6) 



The way in which Maxwell arrived at his equation, as may 

 be inferred from the connexion of his analyses, was based on 



