18 REPOET 1867. 



same potential as that of the air in the neighbourhood of the flame ; and that the 

 efl'ect of a iine jet of -neater from an insulated vessel, is to bring the vessel and 

 other conducting material connected with it to the same potential as th.at of the 

 air, at the point where the jet breaks into drops. In a recent communication to 

 the Royal Societj- "On a Self-acting Apparatus for multiplying and maintaining 

 Electric Charges, with applications to illustrate the Voltaic Theory," an experi- 

 ment was described in which a water-dropping apparatus was employed to prove 

 the difference of potential in the air, in the neighbourhood of bright metallic sur- 

 faces of zinc and copper, metallically connected with one another, which is to bo 

 expected from Volta's discovery of contact electi'icity. In the present communi- 

 cation a similar experiment was described, in which the flame of a spirit lamp was 

 used instead of a jet of water breaking into drops. 



A spirit lamp is placed on an insulated stand connected with a very delicate 

 electrometer. Copper and zinc cylinders, in metallic connexion with the metal 

 case of the electrometer, are alternateh' held vertically in such a position that the 

 flame burns nearly in the centre of the cylinder, which is open at both ends. If 

 the electrometer reading, with the copper cylinder surrounding the flame, is called 

 zei-o, the reading ob^^erved with the zinc cylinder surrounding the flame indicates 

 positive electrification of the insulated stand bearing the lamp. 



It is to be remarked that the different methotls here followed eliminate the 

 ambiguity involved in what is meant by the potential of a conducting system 

 composed partly of flame (alcohol) and partly of metal. In a merely illustrative 

 experiment, which tlie author has already made, the amount of diflerence made by 

 substituting the zinc cylinder for the copper cylinder roimd the flame, was rather 

 more tlian half the difference of potential maintained by a single cell of Daniell's. 

 Thus, when the sensibility of the quadrant divided-ring electrometer * was such 

 that a single cell of Daniell's gave a deflection of 79 .scale-divisions, the difference 

 of the reading, when the zipc 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 Ilank-el, it appears that the diflerence 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 

 DnnielFs. It is quite certain tliat the difference produced in the metal connected 

 with the insulated lamp, would be exactly equal to the true contact diflerence 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 iianie is 

 everywhere sufficient to prevent conduction hj heated air between them, and pro- 

 vided the length of the c}'linder is infinite (or, practically, anything more than three 

 or foui- times its diameter). 



The autlior hopes before long to be able to publi.'-h 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. 



i 



On Electric MacMnes founrled on Induction and Convection. 

 By Sir W. Thomson, LL.D., F.R.S. 



To facilitate the application of an in.strumcnt for recording the signals of the 

 Atlantic cable, recently patented by Sir "W. Tliomson, a small electric machine 

 running easily enough to be driven bj' the wheelwork of an ordinary Morse instru- 

 ment was desired ; and he therefore designed a combination of the electrophorus 

 principle, with the system of reciprocal induction described by liim in a recent 

 comnnmication to the Poyal Societj- (Proceedings, June 1867), which may be 

 briefly described as follows : — 



A wheel of vulcanite with a large number of pieces of metal (called caniers, for 

 brevity) attached to its rim, is kept rotating rapidlj^ roiuid a fixed axis. The car- 

 riers are very lightly touched at ojipusite ends of a diameter by two fixed tangent 

 springs. One of these s.prings (the earth-spring) is connected with the eai'th, and 



* See Proceedings of Eoyal Society, June 20, 1867. 



