189 191] DIELECTRICS AND MAGNETIZABLE BODIES. 367 



depend on the medium, and if the experiment be made in a 

 medium of inductivity /-i the unit thus obtained will be larger in 

 the ratio V//, than if it had been determined in a medium of unit 

 inductivity. We also see that the dimensions of the unit involve 

 fj,, for we must have the dimensional equation 



so that 



It is customary to choose the unit of inductivity so that the 

 inductivity of empty space is unity, or as it is sometimes stated, 

 the inductivity of the ether is unity. This is, as we have seen, 

 purely arbitrary, as experiments enable us to determine only ratios 

 of inductivities. The inductivity of air, both electric and mag- 

 netic, differs very little from that of a vacuum, so that for practical 

 purposes we may consider the size of the units determined by 

 experiments in air. We must notice that even if /z- is put equal 

 to unity its dimensions remain in the equation and the dimensions 

 of fju we have no means of knowing. As the matter of dimensions 

 is always more or less arbitrary, we may make any supposition 

 that we please, until we are led to contradictory results. Two 

 different suppositions are convenient. We may, when dealing 

 with electrical quantities, assume that the dimensions of the 

 electrical inductivity are zero. This gives the electrostatic 

 system of units. We may on the other hand, when dealing with 

 magnetic quantities, assume that the dimensions of the magnetic 

 inductivity are zero. This gives the magnetic system. Both 

 these systems are due to Gauss, and when we use both systems for 

 their respective kinds of quantities, we shall say that the quantities 

 are measured in Gaussian units. This has been the case in the 

 preceding chapters. When we come to deal with both electrical 

 and magnetic quantities at the same time, we must choose one or 

 the other of these assumptions, as we shall find in the next chapter- 

 that both together are incompatible. 



191. Susceptibility. The equation giving the apparent 

 polarization of a medium of inductivity //> surrounded by a medium 

 of inductivity ^ is, ( 186 (30)) 



" N F, 



