1891]. on the Molecular Process in Magnetic Induction. 389 



groups of little magnets, pivoted like compass needles, so that each 

 is free to turn except for the constraint which it suffers on account 

 of the presence of its neighbours. 



But first let us see more particularly what happens when a piece 

 of iron or steel or nickel or cobalt is magnetised by means of a field 

 the strength of which is gradually augmented from nothing. We 

 may make the experiment by placing a piece of iron in a coil, and 

 making a current flow in the coil with gradually increased strength, 

 noting at each stage the relation of the induced magnetism to the 

 strength of the field. This relation is observed to be by no means a 

 simple one : it may be represented by a curve (Fig. 1), and an 



Fig. 1. 



inspection of the curve will show that the process is divisible, 

 broadly, into three tolerably distinct stages. In the first stage (a) 

 the magnetism is being acquired but slowly: the molecules, if we 

 accept Weber's theory, are not responding readily — they are rather 

 hard to turn. In the second stage (6) their resistance to turning has 

 to a great extent broken down, and the piece is gaining magnetism fast. 

 In the third stage (c) the rate of increment of magnetism falls off: 

 we are there approaching the condition of saturation, though the 

 process is still a good way from being completed. 



Further, if we stop at any point of the process, such as P, and 

 gradually reduce the current in the coil until there is no current, 

 and therefore no magnetic field, we shall get a curve like the dotted 

 line PQ, the height of Q showing the amount of the residual mag- 

 netism. 



If we make this experiment at a point in the first stage (a), we 

 shall find, as Lord Rayleigh has shown, little or no residual mag- 

 netism ; if we make it at any point in the second stage (6), we shall 

 find very much residual magnetism ; and if we make it at any point 



Vol. XITI. (No. 85.) 2 d 



