1806.J Electricity produced by the Rontgen Rays, §c. 275 



to begin with, I have not been able to detect that any charge is ac- 

 quired by the plate by exposure to these rays. When the potential 

 to which the plate is raised is high the leakage from the plate is a 

 most delicate means of detecting these rays, more so than any photo- 

 graphic plate known to me. I have found these rays produce dis- 

 tinctly perceptible effects on a charged plate after passing through 

 a zinc plate a quarter of an inch thick. The charged plate and 

 electrometer are nruch more expeditious than the photographic plate 

 and more easily adapted to quantitative measurements. 



To determine how the radiation of the Rontgen rays depended upon 

 the degree of exhaustion of the bulb, the bulb was kept in connection 

 with the pump and the leakage was observed at different degrees of 

 exhaustion ; no leakage could be detected until the pressure was so 

 low that phosphorescent patches appeared on the bulb, and, even after 

 the phosphorescence appeared, the leakage was small as long as there 

 was any considerable luminosity in the positive column ; it was not 

 until this had almost disappeared that the leakage from the charged 

 plate became rapid. 



If the greatest sensitiveness is required, it is, of course, advisable 

 to charge the plate as highly as possible. The leakage due to the 

 rays, however, occurs when the potential of the plate does not exceed 

 that of the tin-plate cover by more than 3 or 4 volts, and I have not 

 yet met with any phenomena which suggest that there is a lower 

 limit of potential difference below which leakage does not take 

 place. 



This leakage differs from that produced by ultra-violet light, the 

 laws of which have been unravelled by Elster and Greitel, in several 

 essential features, in the first place ultra-violet light only discharges 

 a negative charge, while the Rontgen rays discharge both positive 

 and negative. Again, the effect of ultra-violet light is only con- 

 siderable when the electrified body is a strongly electro-positive 

 metal with a clean surface. The effects of the Rontgen rays are, on 

 the other hand, very marked whatever the metal, and take place 

 when the electrified plate is surrounded by solid or liquid insulators 

 as well as when surrounded by air. I have embedded the plate in 

 solid paraffin wax, in solid sulphur, placed it inside a lump of ebonite, 

 wedged it in between pieces of mica, and immersed it in a bath of 

 paraffin oil ; in each of these cases, though the insulation was practi- 

 cally perfect when the insulator was not traversed by the Rontgen 

 rays, and the potential of the plate differed from that of the metal 

 covering of the box by from 10 to 15 volts, yet, as soon as the 

 Rontgen rays passed through the insulator, the charge of the metal 

 plate leaked away. I have found that the electricity leaks from the 

 plate even when the space between it and the nearest conductors con- 

 nected to earth is entirely filled with solid paraffin ; hence we conclude 



