131 



constructed a machine to which the initial charge of mag- 

 netism was imparted by means of a thermo-electric battery. 



The last instance of the repetition of this same idea is 

 that by Sir Charles Wheatstone, in a paper " On the Aug- 

 mentation of the Power of a Magnet by the reaction thereon 

 of currents induced bv the masfnet itself"* 



This enumeration of the instances where the idea of 

 augmenting the force of a magnet by currents induced by 

 itself, the author would have deemed somewhat unneces- 

 sary, were it not that the contrivance had been described 

 as a new principle in electric science, whereas it is, as Mr. 

 Murray justly designates it, an obvious variety of the prin- 

 ciples embodied in the machine the author first described 

 before the Royal Society. 



At the time when this method of exciting an electro- 

 magnet was brought prominently forward by Messrs. 

 Siemens and Wheatstone, the author directed attention to 

 the fact (which would seem to have escaped the notice of 

 these electricians, as they omitted to mention it) that ma- 

 chines constructed as they had described them, are incapable, 

 of themselves, of producing powerful electric currents, as 

 the whole energy of the machine is expended in exciting its 

 own electro-magnet.")* 



While the current transmitted from the armature of a 

 magneto-electric or an electro-magnetic machine through 

 coils surrounding its own magnet is incapable of directly 

 producing powerful electro-dynamic effects, such current 

 may be usefully employed to excite the electro-magnets 

 of other machines in accordance with the author's original 

 method. Some idea of the smallness of the quantity 

 of electricty requisite for this purpose will be found from 

 the fact that the full power of the 10 inch machine is de- 



* Proceedings of the Eoyal Society, vol. xv., p. 369. 



t Proceedings of the Literary and Philosophical Society of Manchester, 

 Tol. vi., p. 103. 



