76 Mr. F. S. Spiers on Contact Electricity. 



When the apparatus was working at its best, and the various 

 adjustments were made with the utmost possible care, a P.D. 

 between the movable plates E and F of one volt caused a de- 

 flexion on the electrometer of about 20 scale-divisions, the 

 sensibility of the electrometer being such that a P.D. of a 

 volt applied directly between its terminals gave a deflexion 

 of 470 scale-divisions. 



It was impossible to increase the size of the plate and thus 

 augment the sensibility of the apparatus, because its dimen- 

 sions were strictly limited by the diameter of the combustion- 

 tubing (A, A, fig. 2). I was fortunate enough to get hold 

 ol a length of 1 J in. diameter. As far as I am aware there was 

 not another piece of combustion-tubing in London of greater, 

 or even as great, diameter. 



§ 8. In the first experiments that were made with this 

 apparatus the dissimilar metals w T ere platinum and zinc. The 

 P.D. between the two, after cleaning the latter with emery- 

 paper and wiping with clean dry silk, was about 0*95 volt. 

 To make measurements in high vacua I first exhausted the 

 tube to a vacuum of about ^ mm. (this took about four 

 hours to obtain), without applying any heat at all. This 

 caused no appreciable reduction of the contact force. (In 

 these experiments the degree of accuracy is not greater than. 

 0*05 of a volt.) The tube was then gently heated with a 

 sinole bun sen-flame, and the pressure soon rose, by reason of 

 the heat driving off occluded or adherent gases from the glass 

 and metal surfaces, to about -^ mm. Long-continued heat- 

 ing with this single bunsen- burner did not noticeably reduce 

 the P.D. between the metals, so an additional source of heat 

 was applied in the shape of another burner; but alter a couple 

 of hours' heating and pumping, although air still continued 

 to come off, no change in the P.D, was observed after the 

 metals were allowed to cool. 



The tube was next heated in a blowpipe flame, the outer 

 copper shield being kept at a red heat for about 10 minutes. 

 Copious air-bubbles now passed through the fall tubes of the 

 sprengel-pump, and the pressure fell from 20 1 00 millim. to 

 -j^. millim. The apparatus was then allowed to cool, the 

 pump being worked continuously. When it was quite cold 

 the pressure was goVo millim., and the P.D. had fallen to 

 about 0T of a volt. It will be seen from § 10 that this was 

 simply the result of oxidation of the zinc, but at the time I 

 thought it might have possibly been a true diminution of the 

 Volta effect due to my having driven off the greater part of 

 the condensed air from the surface of the metal plates. I 

 therefore again heated the tube in the blowpipe flame, this 

 time more strongly than before. Unfortunately the heat 



