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 by the magnet 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 Royal Society, vol. xv., p. 369. 
f Proceedings of the Literary and Philosophical Society of Manchester, 
vol. vi., p, 103. 
