126 ROYAL SOCIETY OF CANADA 



It is possible that further light may be. thrown on these experiments 

 by measurements similar to a series which the writer has recently made 

 in seeking to determine the contact potential diU'ererence between two 

 metals, by using a modification of the apparatus used in the early part 

 of this investigation. 



IV. VoLTA Effect. 



In these experiments the following apparatus was used: 



A circular copper plate AB, about D cm. in diameter, was joined 

 to the electrometer by means of a conductor of the same material, 

 insulated from a surrounding earthed tube by means of amber. A 

 box CC DD', also of copper, served as a guard ring to the plate AB, 

 and as a screen from electrostatic disturbances. Above the plate AB 

 and insulated from it by small ambier supports, a second plate HK of 

 a selected metal was placed, and this was kept joined to earth while 

 experiments with it were being made. Over all a wooden box! MM' NN'^ 

 lined with tin foil, was placed to complete the electric screening. 



With both the plates AB and UK joined to earth, on account of 

 the contact potential difference between them, there would be a current 

 set up between them, if any free ions existed in the intervening space. 

 If, with this arrangement, the earth connection to the lower plate were 

 broken, this plate would, as a result of the current, acquire a charge 

 which would tend to annul the original contact difference. It was 

 therefore thought that, by placing a strong ionizing agent, such as 

 radium, above the plates, the needle would, on breaking the earth con- 

 nection to the lower plate, at once take up a maximum deflection, and 

 60 give a measure of the contact difference. 



It was found that, by using strong radium near the plates, the 

 lower one did acquire in a very few minutes a maximum charge. It 

 was found, too, that as the radium was removed farther and farther 

 from the apparatus, the rate at which this maximum was reached became 

 slower and slower. 



It has been found, however, as a result of the experiments so far 

 performed, that the value of this maximum charge varied, (1) with the 

 material and the tliickness of the upper plate, (2) with the distance 

 between the plates, (3) with the distance of the radium from the plates. 



