Electromagnet and the Law of the Dynamo. 



\ n. 



E. 



e obs. 



e calc. 



i obs. 



i calc. 



j 



850 



0-841 



127 



127 



151-0 



151-0 



853 



1-22 



133 



133 



109-0 



109-0 



855 



2-34 



139 



140 



59 9 



59-4 



850 



452 



138 



139 



30-3 



30-5 



850 



925 



136 



138 



14-7 



14-7 



850 



22-1 



137 



136 



6-2 



6-2 



845 



222 



133 



134 



06 



0-6 



Such a series of results proves the law of the dynamo to be 

 fully established. But if the law of the dynamo, so deduced 

 from Frolich's empirical expression for the electromagnet, be 

 true, then, to an equal degree of precision, must the laiv of the 

 electromagnet be itself true. 



But Frolich's expression is not the expression of any 

 physical law: it is a mere interpolation-formula, destitute of 

 rational significance. What then is the true law of induced 

 magnetism ? And how comes it that an expression devoid of 

 physical significance so nearly expresses the physical law ? 



That question the author believes to have been already 

 solved by a forgotten investigation of Lam on t, published 

 almost without note or comment in his Handbuch von dem 

 Magnetismus in 1867. On p. 41 of that work he gives the 

 following equation: — 



aMx 

 M + ax' 



as a convenient first aproximation to another equation based 

 on a new theory of induced magnetism. In the above 

 equation x stands for the magnetizing force, m the induced 

 magnetism, and M the maximum value of the latter. Writing 



Gk for a, Si for x, and a for ^, 



we at once get 



m = 



GkSi 

 1 + aSi' 



which is the author's way of writing Frolich's equation. 



The physical theory which led Lamont up to this result is 

 of great interest, the more so since it has been revived 

 during the current year in a more elaborate form by Bosan- 

 quet*, who has apparently been led thereto by an inde- 



* Bosanquet, Phil. Mag. ser. 5, xix. p. 85 (Feb. 1885). 



