Measures for Electric and Magnetic Quantities. 383 



the forces exerted by it. These are of two essentially different 

 kinds : — first, the forces independent of its motion, which 

 amounts of electricity always exert upon one another whether 

 they are at rest or in motion; secondly, the forces arising only 

 through the motion. The former are named the electrostatic, 

 and the latter the electrodynamic forces. To the electrodynamic 

 we must reckon the magnetic forces, if we with Ampere derive 

 the explanation of magnetism from small electric currents 

 taking place in the interior of the magnet. Now, of these two 

 forces, we can apply either the one' or the other to the mea- 

 surement of electricity ; and thence arise those two systems 

 of measures, of which the former is called the electrostatic, 

 while the latter (as already said) is ordinarily named the elec- 

 tromagnetic, but would be more rationally called the electrody- 

 namic. In those cases in which it is self-evident that electrical 

 systems of measures are meant, one can, for convenience, 

 omit the prefix "electro," and, briefly, speak of static and 

 dynamic measuring-systems. 



In the electrostatic system the most important unit, which 

 serves as the basis of all the others, is the unit of electricity. 

 This is determined by the following definition: — The unit of 

 electricity is that amount of electricity which exerts the unit of 

 force upon an equal amount of electricity at the unit of distance. 

 We can therefore put the unit of force equal to a fraction 

 having the square of the electricity-unit for its numerator and 

 the square of the length-unit for its denominator. We will 

 at the same time denote, with Maxwell, the electricity-unit by 

 enclosing the letter e in square brackets ; but, in order to ex- 

 press that the static electricity-unit is intended, we will provide 

 the e with the index s, so that the symbol will have the form 

 [e s ~]. The equation in question will then read: — 



M = [MLT-s] ; 



and from this follows 



M = [M*Lt T - 1 ] (1) 



In the electrodynamic system of measures, it would first 

 suggest itself, from what has been said above, to take as the 

 standard that force which, in addition to the static force, two 

 moved quantities of electricity exert upon each other in con- 

 sequence of their motion. Respecting this force, however, 

 differences of opinion prevail, which make the choice of it as 

 the starting-point of the determinations inexpedient. On the 

 other hand, the forces exerted upon each other by closed elec- 

 tric currents can be regarded as indubitably known. As, 

 2G2 



