Section III, 1891. [ 59 1 Trans. Roy. Soc. Canada. 



NOTE 



IX. — Faraday's "Lines of Forced — Suggestion of a Name. 



By Alexander Johnson, M.A., LL.D., Dublin, Professor of Mathematics and Natural 

 "^ Philosophy, McGill University, Montreal. 



(Read May 27, 1S91.) 



It must be confessed that the introduction of a new name into science is a matter of 

 great difficulty, but though the success of the attempt must be very doubtful, yet in the 

 present case the need is so pressing and the confusion arising from the want of a proper 

 term so great, that the attempt may at least incite others to the production of some term 

 that may meet with general acceptance. 



The term " lines of force " is used in two different senses, but this is not the strongest 

 objection to it. In ordinary language, the use of the same word in two distinct mean- 

 ings is often unavoidable, though always to be regretted ; in the exact sciences the exist- 

 ence of such a term is rightly regarded as a positive blot, yet it seems to be sometimes 

 unavoidable also. For example, we have the term " pole " in spherical geometry, in 

 plane geometry and in physics ; but this, owing to the difference of the subjects, leads to 

 no confusion, being employed steadily in the same sense throughout any one scientific 

 paper or investigation. On the contrary, the term " lines of force " may be used in differ- 

 ent senses in the same page, and one of these, to the utter confusion of students, contra- 

 dicts one of the fundamental notions of geometry. 



The liability to error is so great that Faraday himself, who introduced the terra, 

 could not avoid the confusion. "When he first employed it he defined these lines, for 

 magnetic force, as the curves to which a very small magnetic needle would be a tangent, 

 or as those which would be depicted by iron filings. He at the same time wished to use 

 them to represent the magnetic power, " not merely in the points of quality and direction, 

 but also in quantity." In the twenty-eighth series of his ' Experimental Researches ' he 

 says : " A point equally important to the definition of these lines is that they represent a 

 determinate and unchanging amount of force. Though, therefore, their forms, as they 

 exist between two or more centres or sources of magnetic power, may vary very greatly, 

 and also the space through which they may be traced, yet the sum of power contained in 

 any one section of a given portion of the lines is exactly equal to the sum of power in any 

 other section of the same lines, however altered in form, or however convergent or diver- 

 gent they may be at the second place." He considered that the employment of these 

 lines would in many cases have a great advantage over the method which treated the 



