370 



Mr. J. Brown. Some Experiments 'bearing 



condensed on their surfaces from the atmosphere (vapour or gas) in 

 which the metals are immersed. From this view it would naturally 

 follow that, if all chemically active films and all atmosphere competent 

 to produce them could be removed, the difference of potential would 

 disappear also. This inference has been pointed out and acted upon 

 by previous experimenters, as is well known ; but it appeared to me 

 that no certain test of the theory could be obtained in the way they 

 suggested without elaborate precautions as regards details ; and the 

 experiments here to be described were undertaken in the hope that 

 more definite results might follow greater care in the work. 



The method adopted was to enclose a copper-zinc volta condenser in 

 a glass tube containing nitrogen at a small pressure, together with 

 metallic potassium and sodium, the expectation being that these metals 

 would absorb any remaining water vapour 

 or other agents (compounds of oxygen, &c.) 

 that could exert chemical action on the zinc. 

 The condenser is represented in the figure. 

 Its plates are 101 mm. long by 47 mm. wide. 

 The copper plate C has prolongations at 

 both ends, to which are attached the fittings 

 D E, and the springs E S, for the support of 

 the whole system in its glass tube. To the 

 prolongation A of the copper plate is hinged, 

 on pointed screws, a brass sleeve, into which 

 is cemented the glass plate G, as an insula- 

 ting support for the zinc Z. The free end of 

 the glass plate hangs in a notch in the fitting 

 E, and can move to the extent of the width 

 of this notch, so as to separate the plates 

 when the condenser with its tube is tilted 

 over till the zinc falls away from the copper. 

 Two curved springs of strip steel are placed 

 between the glass and the zinc, to keep them 

 apart ; while they are held together by three 

 screws, passing through the glass, and about 

 half way through the zinc. These screws 

 also serve to keep the copper and zinc, when 

 in their position of nearest approach, at the 

 uniform distance of about 0'03 mm. 



The surfaces of the places were made 

 true by careful filing, scraping, and testing 

 by a surface plate. Platinum-tipped contact 

 springs, P V, are provided to make connec- 

 tion with platinum wires sealed into the 

 containing tube, which is of lead 



