Mr. J. P. Joule's Account of Experiments 



particle of space through whicli a certain quantity of electricity 

 is propagated, appears to operate in moving the magnetism of 

 the bar with a force proportionate to the inverse square of its 

 distance from the iron j and that when the tension or specific 

 magnetism is the same, the thickness of the iron upon which 

 that particle of conducting space acts, has nothing (apart from 

 resistance and other foreign circumstances) to do with the whole 

 effect. Now it may be mathematically demonstrated, that, such 

 being the law, if each pai'ticle induce upon a larffe surface, the 

 resulting magnetic force will not vary mucli with the distance, 

 but be a very constant quantity for any distance which bears a 

 small ratio to the dimensions of that surface. Hence it is that 

 a coil within a hollow piece of iron has no power to magnetize 

 it*. And hence also in the case of my large electro-magnet, of 

 which the surfaces are large, eveiy particle of conducting wire 

 would perform its full extent of duty, even if it were not quite 

 close to the iron. 



"\ATien the interferences arising from tension are reduced to a 

 minimum by completing the magnetic circuit and making use 

 of a very small electrical force, the resistance from length becomes 

 a veiy sensible quantity, varying probably in the direct ratio of 

 that element. Some idea of its character may be fonned from 

 the following table, in which I have compared half the maximum 

 powers of each electro-magnet with the electro-magnetic forces 

 which produced them ; and by dividing the former by the latter 

 I have obtained the third column, which, under the title of spe- 

 cific power, contains the quantity of lifting power (of that degree 

 of tension) due to an unit of electro-magnetic force. 



Table V. 



The electro-magnetic force against No. 2 is rather greater 

 than the truth, on account of the greater relative distance of its 

 coils from the iron; allowing for this, we may observe that the 

 variation of the specific powers is due to the resistance of length. 



It is well knoAvn, that, after the current is cut off from an 



electro-magnet, the ai'mature is retained in its place with very 



considerable force. I was anxious to try the capability of my 



electro-magnet No. 1 in this respect, and have arranged the 



* Scientific Memoirs, part 5. p. 14. 



