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



an incli square, with tlie attractions of electro-magnets 30 indies 

 long and of the sections 1 inch square and 2 inches hy 1. In 

 these experiments the poles of the attractive magnets were |fth 

 of an inch distant from one another, and it is probable that the 

 deterioration of the attractive energy of the long electro-magnets 

 was principally owing to the greater extent of their polar surfaces. 

 , Hence also corrections ought to he applied to the attractions 

 of the larger electro-magnets in Tables I. and II. of my previous 

 paper, in order fairly to compare their respective powers with 

 those of the smaller magnets. 



These corrections would not, however, be of sufficient amount 

 to affect the general conclusion at which I have arrived, with 

 regard to the laws under which magnetic attraction (as applicable 

 to the production of motive force) is developed by electricity, 

 viz. that the attraction of two electro-mo gnets towards each other 

 is in every case rqiresented by the formula M = W'2E^, where M 

 denotes the magnetic attraction, W the length of wire, and E 

 the quantity of electricity conveyed hy the wire in a given period 

 of time ; a formula modified merely by the effects of approaching 

 saturation, of the conducting power of the iron, and of the di- 

 stance of the coils from the surface of the iron. 



I have observed that magnetic and electro-magnetic attractions 

 decrease in certain cases in the simple ratio of the distances. 

 This was found to be the case, particularly when the magnets 

 were long and the distance between them small. Mr. Harris 

 has noticed the same phcenomenon in his " Experimental Inquiries 

 concerning the laws of Magnetic Forces.^' It may be almost en- 

 tirely accounted for by the complex action previously illustrated. 

 It is impossible to doubt, that the law of magnetic attraction is 

 that of the inverse square of the distances. 



?!< ^ ^ Jji >(» 'P 



Broom Ilill, near Manchester, 

 March 10, 1840. 



On Electro-Magnetic Forces*. 



In resuming the relation of my researches, I shall consider 

 the laws which govern that peculiar condition which is assumed 

 on the completion of the ferruginous circuit, viz. the lifting or 

 sustaining power of the electro-magnet. 



Although this wonderful property is known to all, and a 

 variety of forms have been given to the electro-magnet both as 

 regards the bulk and shape of its iron and the length and num- 

 ber of its spirals, I am not aware that any general rules have 

 been laid down for its manufacture. I shall therefore attempt 

 to supply this want, and in so doing shall describe a construction 

 attended by far gi'cater results than have hitherto been produced. 

 * Annalsof Electricity, vol. V. p. 187. 



