442 Mr. H. Wilde on some Improvements in 



That this contrivance suggested itself independently in dif- 

 ferent countries to the several experimenters above mentioned, 

 there is as little reason to doubt as that the similar animistic ideas 

 and customs found amongst the primitive races in various parts 

 of the world are of independent origin ; but such repetitions ex- 

 clude the contrivance from the rank of a discovery in science ; 

 and it is, as Mr. Murray justly designates it, an obvious variety 

 of the principles embodied in the machine I first described before 

 the Royal Society. 



At the time when this method of exciting an electromagnet 

 was brought prominently forward by Messrs. Siemens and 

 Wheatstone, I directed attention to the fact (which would seem 

 to have escaped the notice of these electricians, as they omitted 

 to mention it) that machines constructed as they had described 

 them are incapable by themselves of producing powerful electric 

 currents, as the whole energy of the machine is expended in ex- 

 citing its own electromagnet*. Besides this, the actual amount 

 of electricity circulating round the electromagnet is really very 

 small ; and when the circuit is opened for the purpose of applying 

 the current to some useful purpose, the magnet immediately 

 discharges itself and resumes its neutral condition till the con- 

 tinuity of the metallic circuit is reestablished. 



While the current transmitted from the armature of a mag- 

 neto-electric or an electromagnetic machine, as I have said, is 

 incapable of directly producing powerful electrodynamic effects, 

 such current may be usefully employed to excite the electromag- 

 nets of other machines in accordance with my original method. 

 Some idea of the smallness of the quantity of electricity re- 

 quisite for this purpose will be formed from the fact, that the 

 full power of the 10-inch machine is developed when its electro- 

 magnet is excited by the current from four pint Grove's cells. 



This machine has been in constant operation for some time 

 past for the electro-deposition of metals from their solutions ; 

 and its electromagnet is now excited by its own residual magne- 

 tism in the following manner: — A small magnet-cjdinder (3*5 

 inches diameter and 14 inches long) is bolted to the top of the 

 10-inch cylinder, so that the sides and axis of the former are pa- 

 rallel with the similar parts of the latter. The cylinders are 

 separated for a space of three quarters of an inch by a packing of 

 brass, and consequently act upon each other by induction through 

 the intervening space, instead of by contact as in ordinary me- 



Theories, delivered at the Royal Institution of Great Britain, 1870, by John 

 Tyndall, LL.D., F.R.S., " desired by persons interested in education." 



* Proceedings of the Literary and Philosophical Society of Manchester, 

 vol. vi. p. 103. 



