Sir W. Thomson on Volt a- Convection by Flame. 65 



had pointed out that the effect of the flame of an insulated lamp 

 is to reduce the lamp and other conducting material connected 

 with it to the same potential as that of the air in the neighbour- 

 hood of the name, and that the effect of a fine jet of water from 

 an insulated vessel is to bring the vessel and other conducting 

 material connected with it to the same potential as that of the 

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

 communication to the Royal Society " On a Self-acting Appa- 

 ratus for Multiplying and Maintaining Electric Charges, with 

 applications to illustrate the Voltaic Theory," an experiment 

 was described in which a water-dropping apparatus was employed 

 to prove the difference of potential in the air, in the neighbour- 

 hood of bright metallic surfaces of zinc and copper metallically 

 connected with one another, which is to be expected from 

 Volta's discovery of contact-electricity. In the present commu- 

 nication a similar experiment is described, in which the flame of 

 a spirit-lamp is 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 alternately held vertically 

 in such a position that the 

 flame burns nearly in the cen- 

 tre of the cylinder, which is open 

 at both ends. If the electro- 

 meter-reading, with the copper 

 cylinder surrounding the flame, 

 is called zero, the reading ob- 

 served with the zinc cylinder 

 surrounding the flame indicates 

 positive electrification of the in- 

 sulated stand bearing the lamp. 



It is to be remarked that the 

 differential method here follow- 

 ed eliminates the ambiguity in- 

 volved in what is meant by the 

 potential of a conducting system 

 composed partly of flame, partly 

 of alcohol, and partly of metal. 

 In a merely illustrative experiment, which the author has already 

 made, the amount of difference made by substituting the zinc 

 cylinder for the copper cylinder round the flame was rather 

 more than half the difference of potential maintained by a single 



Phil. Mag. S. 4. Vol. 35. No. 234. Jan. 1868. F ' 



