Pole in the Electrostatic System of Units. 365 



molecules there is an extra magnetic induction, due to the 

 pointing of these along the lines of force, which is 4-7T times 

 the magnetization, and which has to be added to the other, 

 thus making the total magnetic induction at any point fi times 

 the magnetic force there. 



The effect of the medium is a physical fact; and no theory 

 can presume really to dispense with the constant fi. All that 

 the Amperian theory does is to give a physical interpretation 

 to it, and to render one independent of it so soon as one takes 

 account of every current-conveying circuit, whether molecular 

 or other, existing in the field, and does not arbitrarily elect to 

 deal only with those gross solenoids which we can excite and 

 immediately control by batteries. 



There can be no doubt, I think, that the mind of Maxwell 

 on this subject, as on most others, was as clear as daylight ; 

 and so far from falling into the least suspicion of an error, he 

 expresses himself in art. 615 (1st edit.) almost as if he were 

 joining in the present discussion, saying : — 



" There is one result .... which is of very great importance. 

 If we suppose that no magnets exist in the field except in the 

 form of electric circuits, the distinction which we have hitherto 

 maintained between the magnetic force and the magnetic 

 induction [and therefore also the difference /a — 1] vanishes, 

 because it is only in magnetized matter that these quantities 

 differ from each other. According to Ampere's hypothesis 



the properties of what we call magnetized matter are 



due to molecular electric currents, so that it is only when we 

 regard the substance in large masses that our theory of mag- 

 netization is applicable ; and if our mathematical methods are 

 supposed! capable of taking account of what goes on within 

 the individual molecules they will discover nothing but elec- 

 tric circuits, and we shall find the magnetic force and the 

 magnetic induction everywhere identical. In order, however, 

 [N.B.] to be able to make use of the electrostatic or of the 

 electromagnetic system of measurement at pleasure we shall 

 retain the coefficient /n, remembering that its value is unity 

 in the electromagnetic system." 



I am, Gentlemen, 



Your obedient servant, 



University College, Liverpool* OLIVER J. LODGE, 



September 28, 1882. 



