PRESIDENT S ADDRESS SECTION A. 



47 



^ dP 

 In the displacement theory, rf = — E ~— ~ and v'P = '*• 



ci o 



Similar laws hold of the anologues oi' d, E, and P. 



And now I have ended the task which I set myself. I 

 have tried to show briefly the extent of the analogy between 

 all these different theories. I believe it is most important that 

 every physical student should examine this analogy, because 

 in doing so he looks at each theory from many points of view, 

 and, even more, because he learns to substitute in all the 

 theories the idea of continuous action through some medium 

 for that of action at a distance. The tendency of modern 

 work is always to make this substitution : no better example 

 can be found than the growing use of the idea of the 

 magnetic circuit. It is as though — I am supposing that even 

 now you are not quite tired of analogies — the pioneers of 

 electricity had slowly and painfully made their way, by 

 tortuous paths, through country new and difhcult. We, 

 following in their footsteps, and learning to know the country 

 they rendered accessible, have found that by starting from a 

 somewhat different base we may attain the same ends by 

 straighter and plainer roads. With these new roads it is our 

 duty to become familiar. We shall be rewarded for our pains 

 by finding ourselves able to make a fresh start — a further 

 advance into regions as yet unknown. 



