THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



DECEMBER 1888. 



LIV. Note on the Conditions of Self-Excitation in a Dynamo 

 Machine. By Silvanus P. Thompson, D.ScJ* 



WHEN about the year 1867 it began to be recognized 

 that a machine constructed on the principle of magneto- 

 electric induction might be made to excite the magnetism in 

 it? own field-magnets, it became usual to explain the pheno- 

 menon of self-excitation in language something like the 

 following. There being a small amount of residual mag- 

 netism in the core of the stationary electromagnet, very feeble 

 currents will be induced in the secondary or armature-coils 

 when they are set into rotation at any given speed. If these 

 feeble induced currents are now sent round the coil of the 

 fixed electromagnet they will exalt its feeble magnetism, and 

 its inductive action will be stronger ; these stronger currents 

 being also sent round the magnet will raise its magnetism still 



o o o 



higher, and so by action and reaction the magnetism and the 

 currents grow at a compound-interest rate until the limit fixed 

 by the magnetic saturation of the iron core of the field-magnet 

 is reached. 



Long before the true rationale of the limit of self-excitation 

 was assignable, it was known that this method of considering 

 the subject was faulty. Were it true, it would follow that, 

 no matter what the speed of driving, or the resistance in the 

 electric circuit, the magnetism would go on rising until the 

 iron had attained saturation. The facts, on the contrary, are 

 (1) that in self-exciting machines the degree of excitation of 

 the field-magnets depends most emphatically on the speed and 



* Communicated by the Physical Society : read May 12, 1888. 

 Phil Mag. S. 5. Vol. 26. No. 163. Dec. 1888. 2 K 



