MODERN PHYSICS. 185 



force and electric displacement in a dielectric; (iii) 

 that between magnetic force and the current which 

 gives rise to it And we have seen that an electric 

 current i.e. in a dielectric the variation of the 

 strength of an electric field of force gives rise to 

 magnetic force. Now, magnetic force acting on a 

 medium produces " magnetic displacement," or mag- 

 netic induction, as it is called In all media except 

 iron, nickel, cobalt, and a few other substances, the 

 magnetic induction is proportional to the magnetic 

 force, and the ratio between the magnetic induction 

 produced by a given force and the force is found to 

 bo very nearly the same for all such media. This 

 ratio is known as the permeability, and is generally 

 denoted by the symbol /*. 



A relation reciprocal to that given in (iii) above 

 might be anticipated, and was, in fact, discovered by 

 Faraday. Changes in a field of magnetic induction 

 give rise to electric force, and hence to displacement 

 currents in a dielectric or to conduction currents in 

 n conductor. In considering the relation between 

 these changes and the electric force, it is simplest 

 at first not to deal with magnetic matter such as 

 iron, nickel, or cobalt ; and then we may say that (iv) 

 the work which at any instant would be done in 

 carrying a unit quantity of electricity round a 

 closed circuit in a magnetic field against the electric 

 forces duo to the field is equal to the rate at which 

 the total magnetic induction which threads the 

 circuit is being decreased. This law, summing up 

 Faraday's experiments on electro-magnetic induction, 

 gives a fourth principle, leading to a fourth series 



