1896.] X Rays on the Contact Electricity of Metals. 337 



quite balanced by the electrolytic action. This is confirmed by the 

 difference between 057 and — 0'54 given above. After a short while 

 the deflection fell to 



-0-48 volt, 



which is nearly the value found previously for untarnished zinc and 

 tinfoil in air, showing that the acidulated water has, in all prob- 

 ability, removed the tarnish, and is now balancing the contact 

 potential between clean surfaces of the metals. The water was now 

 removed, and the contact potential measured by the null method as 

 before. It is still 



+ 0-57 volt, 



for, of course, by far the greater part of the surfaces of the plates is 

 still tarnished as before. In another experiment with a drop of 

 acidulated water between the plates, I found that the contact poten- 

 tial was more than counter-balanced by the electrolytic action. This 

 corresponds to the action of the rays mentioned in § 7. 



§ 12. It now occurred to me, that perhaps the electrolytic con- 

 nexion established between the plates when the rays were passing 

 might be through the insulating supports of the tinfoil plate, and 

 not through the air, as I had hitherto supposed. To test the truth of 

 this idea, several variations were made in the arrangements, all with 

 a view to place the insulators in such a position that the rays could 

 not reach them. First, I placed a screen of sheet zinc, with a hole in 

 the middle of it, between the tinfoil plate and the hole in the box, 

 from which the rays issue, so that the rays from the negative elec- 

 trode of the vacuum bulb could fall only on the tinfoil plate and not 

 on its insulating supports. These, by the way, consisted of pieces of 

 good red sealing-wax, and, under ordinary conditions, were excellent 

 insulators. When the rays were started the electrometer moved from 

 zero to a position which indicated a difference of potential as great as 

 or greater than any which had been previously observed. This shows 

 that the electrolytic connexion had not been at all affected by screen- 

 ing the solid insulators. 



This experiment was repeated several times, and in every case the 

 result was the same. Still further confirmation of the aerial nature 

 of the connexion was obtained by a great alteration in he length and 

 position of the insulating support. Instead of three short arches of 

 sealing-wax, each made of one stick about 6 inches long, I used one 

 tall one fully 12 inches high ; one leg was attached to the wooden 

 framework on which the uninsulated zinc plate was mounted, and the 

 other, which was somewhat longer, held the tinfoil plate. The new 

 insulator was thus one piece, about 24 inches long, of sealing-wax, 

 with the greater part far removed from the rays, instead of six 

 pieces each 3 inches long. A number of experiments were made with 



