66 Sir W. Thomson on an Electric Machine 



cell of DanielPs. Thus, when the sensibility of the quadrant 

 divided-ring electrometer* was such that a single cell of DanielFs 

 gave a deflection of 79 scale-divisions, the difference of the 

 reading when the zinc cylinder was substituted for the copper 

 cylinder round the insulated lamp was 39 scale-divisions. 

 From other experiments on contact-electricity made seven years 

 ago by the author, and agreeing with results which have been 

 published by Hankel, it appears that the difference of potentials 

 in the air in the neighbourhood of bright metallic surfaces of 

 zinc and copper in metallic connexion with one another is about 

 three-quarters of that of a single cell of DanielFs. It is quite 

 certain that the difference produced in the metal connected with 

 the insulated lamp would be exactly equal to the true contact 

 difference of the metals, if the interior surfaces of the metal 

 cylinders were perfectly metallic (free from oxidation or any 

 other tarnishing, such as by sulphur, iodine, or any other body); 

 provided the distance of the inner surface of the cylinder from 

 the flame were everywhere sufficient to prevent conduction by 

 heated air between them, and provided the length of the cylinder 

 were infinite (or, practically, anything more than three or four 

 times its diameter). 



The author hopes before long to be able to publish a complete 

 account of his old experiments on contact-electricity, of which a 

 slight notice appeared in the Proceedings of the Literary and 

 Philosophical Society of Manchester. 



VIII. On Electric Machines founded on Induction and Convection. 

 By Sir William Thomson, LL,D., F.R.S.f 



TO facilitate the application of an instrument, which I have 

 recently patented, for recording the signals of the Atlantic 

 Cable, a small electric machine running easily enough to be 

 driven by the wheelwork of an ordinary Morse instrument was 

 desired; and I have therefore designed a combination of the 

 electrophorus principle with the system of reciprocal induction 

 explained in a recent communication to the Royal Society (Pro- 

 ceedings, June 1867), which maybe briefly described as follows: — 

 A wheel of vulcanite, with a large number of pieces of metal 

 (called carriers, for brevity) attached to its rim, is kept rotating 

 rapidly round a fixed axis. The carriers are very lightly touched 

 at opposite ends of a diameter by two fixed tangent springs. 

 One of these springs (the earth- spring) is connected with the 



* See Proceedings of the Royal Society, June 20, 1867. [Phil. Mag. 

 November 1867, p. 391.] 



f Communicated by the Author. 



