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XLVT. JSotes on Electricity and Magnetism, — 1 . The " Rails 

 and Slider " Magneto-machine. 2. Permanent Magnets and 

 Dynamical Theory. By Professor A. Gray, F.R.S.* 



VERYONE knows the arrangement of rails and sliding 

 bar in a uniform magnetic field which constitutes the 

 simple mageto-machine devised in 1863, or thereabouts, by 

 Lord Kelvin to illustrate the dimensions (first pointed out 

 I believe by Wilhelm Weber) of electric resistance in the 

 ordinary electromagnetic system of units. The first account 

 of it, so far as I am aware, is in the 1863 Report of the 

 B. A. Committee on Electrical Standards, where it appears 

 twice, first in the general part of the Report, and again in 

 § 30 of the Appendix on " Elementary Relations between 

 Electrical Measurements," which was drawn up by Clerk 

 Maxwell and Fleeming Jenkin. It is very curious that 

 in these first statements of the action of the machine, and 

 all later discussions of it (including my own) which I have 

 seen, the theory given is imperfect in one important respect f . 

 No mention whatever is made of the self-inductance of the 

 circuit; and it does not seem possible by any special arrange- 

 ment of the conductors to make the self-inductance remain 

 zero or constant as the slider moves. In the references to 

 the arrangement in Maxwell's 'Electricity,' §§ 594, 595, 596, 

 the question of self-inductance does not arise. 



At the date mentioned self-inductance was no doubt much 

 less in the minds of electricians than it is now. For example,, 

 though it plays no part in the mathematical theory of current 

 induction sketched by Helmholtz in 1847, in his famous- 

 essay " Die Erhaltung der Kraft," its existence had been 

 fully established by Faraday's experiments ('Experimental 

 Researches,' § 1090, &c): and it was the great feature of 

 Thomson's paper published in this Journal in 1853, that the 

 term ^L7" in the elect rokinetic energy of a circuit (explicitly 

 pointed out in a previous paper " On the Mechanical Values 

 of Distributions of Electricity, Magnetism, and Galvanism/'" 

 communicated to the Glasgow Philosophical Society in 

 January of that year) is applied to the explanation of the 

 oscillatory discharge of a condenser through a coil of wire 

 connecting its plates. Here the self-inductance appears 

 under the name of " Electromagnetic Inertia," which very 

 clearly indicates its dynamical significance. 



* Communicated by the Author. 



t (Note added Feb. 12. ^ Professor Carey Foster has kindly called 

 my attention to the fact that the omission is noticed in his and Prof. 

 Porter's < Electricity and Magnetism,' editions 1909, 1913, § 299. 



