254 Mr. E. Taylor Jones on 



be indicated that there is a certain value of L which produces 

 the most efficient result. 



If L = 0, W equals 0; if L = co, W equals zero ; so that 

 there is some value of L between and go which it is best 

 to employ. 



The actual problem, which I have not gone into, is more 

 complicated still ; for both K and L depend on the winding : 

 for precise treatment, a certain type of winding might be 

 adopted, and K and L made to depend on some variable 

 depending on the character of the winding ; perhaps expe- 

 riments would give results more readily than mathematics. 



Addendum III. 



Each resistance of variable self-induction was made of two 

 bobbins, one of which rotated within the other, after the 

 manner of those described by Lord Rayleigh in Phil. Mag. 

 1886, xxii. p. 473. 



To compare and graduate the self-inductions I used a method 

 which was very convenient and sensitive, a modification of 

 that described in Clerk-Maxwell, vol. ii. art. 757. 



The adjoining figure represents a 

 Wheatstone's bridge; the letters have 

 their usual significance. Let self-in- 

 duction of AD = L, of CD = L'. 



P, Q, R, S were arranged so that 

 with a steady current and galvanometer 

 no current went through the galvano- 

 meter. 



The galvanometer was then replaced 

 by a telephone, the steady by an alter- 

 nating current, and the rotating bobbin of one of the resist- 

 ances was rotated until no noise was heard in the telephone. 

 In this case 



L = R = P 

 L' S Q' 



XXIV. On Electromagnetic Stress. By E. Taylor Jones, 

 B.Sc, Science Scholar of Royal Commission for Exhibition 

 of 1851, nominated by the University College of N. Wales*. 



THE problem of finding experimentally the true relation 

 between electromagnetic stress, or " lifting-power n per 

 unit area of magnets, and magnetization has been attacked 

 by many experimenters during the last sixty years; but no one 



* Communicated by Lord Playfair, F.R.S. 



