THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



JANUARY 1886. 



I. On the Law of the Electromagnet and the Law of the 

 Dynamo. By Silvanus P. Thompson, D.Sc., B.A.* 



IT is remarkable, considering the vast amount of research 

 that has been made, that authorities in physics are in so 

 little agreement as to what the law of the electromagnet is. 

 Upon the one topic that is all-essential to the electric engineer 

 the professed physicist is either silent, or gives mathematical 

 rules that are hopelessly wide of the observed facts, and based 

 in most cases on no consistent; theory. The expressions given 

 by Weberf for the relation between the magnetizing force 

 and the induced magnetism are an exception ; but they are 

 unmanageable in the extreme, and have led to no useful re- 

 sults. The expression given by LamontJ is also based on a 

 certain consistent theory, but it appears to be entirely un- 

 known both to physicists and to electric engineers, and is to 

 be found only buried in the depths of Lamont's own treatise. 

 The two mathematical formulas which are most often given in 

 treatises on physics as representing the relation between the 

 magnetizing current and the induced magnetism of the elec- 

 tromagnet are the following : — 



Lenz and Jacobis Formula. — According to the experiments 

 of these early investigators (1839), the magnetism of the 

 electromagnet is simply proportional to the strength of the 

 current and to the number of turns of wire in the coil. This 

 may be written as m~k$i 



* Communicated by the Physical Society : read 14th Noyember, 1885. 

 t Weber, Elektrodynamische Maasbestimmungen, p. 572. See also 

 Maxwell, ' Electricity and Magnetism ' (2nd edition), yol. ii. p. 78. 

 \ Lamont, Magnetismus, p. 41. 



Phil. Mag. S. 5. Vol. 21. No. 128. Jan. 1886. B 



