230 MAGNETISM 



great discovery that when polarised light is passed through certain 

 media along the lines of magnetic induction, the plane of polarisation 

 is rotated, a discovery clearly proving that in these media the 

 altered condition is no mere hypothesis, but a certain fact, lie also 

 laid emphasis on the idea, which, however, was not a new one, that 

 the magnetic condition of a magnet is not merely a condition on 

 its surface or at its poles, but that there is something going on inside, 

 something symbolised by the passage of lines of induction through 

 it. We are driven to this conclusion by observing that when \u- 

 break a magnet in two, new poles are developed on the broken 

 ends. The change in length of iron bars on magnetisation, a change 

 proportional to the length, is perhaps further evidence of some 

 action or some change of condition within the metal. 



It may appear to be a difficulty in the way of accepting the action 

 by and through the medium that the action of a magnet is 

 practically the same through or in most media surrounding it. It 

 is very nearly the same whether the magnet is surrounded by water 

 or by air, or whether it is placed in a vacuum. We must then 

 suppose that there is some medium even in a vacuum which is 

 altered by the presence of the magnet, that there is something 

 which can transmit momentum and which can ]><>-M ^ cm: 

 But this difficulty has to be fa.ced in considering the phenomena <f 

 radiation. Radiant energy passes at very nearly the same speed 

 through air and through the highest vacuum \\hich we can create, 

 and it passes through the vacuum, as far as we yet know, without 

 any loss of energy. There is something, then, in the vacuum which 

 can take up and propagate radiant energy and its accompanying 

 momentum, and nearly in the same \\ay as if air were present. 

 When we speak of a field in air, we must not, then, imply that tin- 

 air is the seat of all the action, the storer of all the cncrgv. 

 Doubtless it takes some share in the action, but the medium which 

 pervades a so-called vacuum is no doubt present, and in all proba- 

 bility takes its share in the action. 



Induction and intensity in air. When magnetisation 

 of iron takes place by induction, we symbolise it by supposing that 

 lines of induction pass through the iron, and we say that induction 

 goes on within it. We may think of the induction within as 

 implying a condition in the iron similar to that which exists in 

 the neighbouring air outside, but in general the action, whatever it 

 is, will be greater. The term induction is therefore extended to 

 denote the alteration existing in the air, as well as that in iron. 



In the air, or in any non-magnetic medium, we take the 

 intensity H, the force on a unit pole, to represent the induction, 

 both in magnitude and direction. In doing this we make no 

 hypothesis as to the nature of the induction. All that we imply is 

 that the inductions at two points P and Q in air will differ from 

 each other in magnitude and direction in the same way as the 

 intensities at P and Q, and we so choose the unit of induction 



