584 ELECTRICAL UNITS. 



CHAPTER IX. 

 ELECTRICAL UNITS. 



603. FUNDAMENTAL UNITS. DERIVED UNITS. In the phe- 

 nomena of electricity and magnetism experimenters have for a 

 long time evaluated the various quantities only as functions 

 of arbitrary units, the choice of which was determined in each 

 case by the convenience of experiments. This method, even when 

 the units employed were suitably denned, has the inconvenience 

 not only of making it very difficult to compare the results obtained 

 by different observers, but particularly of masking the relations 

 which may exist between various orders of phenomena. It is then 

 of the greatest importance, for the progress of science, to arrive 

 at a common understanding as to the choice of the units, and at 

 the same time that the units adopted shall have that character of 

 co-ordination amongst themselves which constitutes the superiority 

 of the metrical system. 



The units corresponding to the different kinds of magnitudes 

 may, in fact, be chosen arbitrarily, and independently of each 

 other ; but there is an obvious advantage in making them depend 

 on as small a number of simple units as possible. Thus, in 

 geometry, the unit of surface and the unit of volume may be 

 derived from the unit of length. In Kinematics we introduce 

 with the velocity a new idea and a new unit that of time. The 

 study of dynamics leads to a third unit, independent of the two 

 former the unit of force or the unit of mass. All mechanical 

 magnitudes may thus be evaluated as functions of the three units 

 of length, of time, and of mass, or of length, of time, and of force. 

 In a co-ordinated system, the irreducible units are called fundamental 

 units ; the others are called derived units, 



All magnitudes which we deal with in electricity and magnetism 

 have been denned by their mechanical properties ; they may 

 therefore be measured, like the mechanical quantities themselves, 



