ELECTROMAGNETIC UNITS. 439 



pressed by the same number as the magnetic power of the equivalent 

 shell of the same contour. We deduce from it 



k 

 We shall see later what is the significance of this factor - . 



2 



With the new expression for the intensity, the action of an un- 

 limited rectilinear current at the distance a becomes 



(12) 



Consequently, the electromagnetic intensity equal to unity is that 

 of the unlimited rectilinear current which at unit distance exerts a 

 magnetic force equal to 2. 



461. ELECTROMAGNETIC UNITS. The change in the expression 

 for the strength necessarily leads to corresponding modifications in 

 estimating other electrical quantities. If it is desired that the in- 

 tensity shall always represent the quantity of electricity which 

 traverses a section of a conductor in unit time, the equation 



will define Q, and therefore the unit of electricity. 



If the current produces no other work than that of heating the 

 circuit, Joule's law will define the expression of the resistance, and 

 therefore the unit of resistance, by the ratio 



W = PR/, 



in which W represents the thermal energy produced during the 

 time /. 



The electromotive force, lastly, is given by the equation 



W-EI/. 



The units thus defined, and which we shall call electromagnetic 

 units, are those used in the following chapters. We shall subse- 

 quently establish the relations between them and the electrostatic 

 units. 



