356 Prof. J. J. Thomson : Experiments on 



The rods which had been made active by long negative 

 electrification gradually lost this activity, and after the lapse 

 of about one hour the current with these rods as electrodes 

 sank to about its normal value. When the rod in the gas 

 exposed to the Rontgen rays was either positively electrified 

 or not electrified at all, no change took place in the satura- 

 tion-current sent through the gas before and after the 

 electrification from this wire as electrode, thus in these 

 experiments, as in those of Elster and Geitel, negative electrifi- 

 cation is required to make the rod active. 



When a rod is in the active state, the current in the 

 direction corresponding to a flow of positive electricity from 

 the rod is slightly greater than the current in the opposite 

 direction. When the rod is in the normal state the two currents 

 are equal. The effects produced on the rods in the preceding- 

 experiments are not very great ; but the method described 

 below gives very large effects, exceeding even those produced 

 by electrification in the open air. 



Properties of Air Bubbled through Water. 



I made further experiments to see whether the induced radio- 

 activity on negatively electrified bodies could be detected in 

 the space of an ordinary- sized room without artificial ioniza- 

 tion of the air. I endeavoured to produce throughout the air 

 in the room an electric field of greater average intensity 

 than that produced by a negatively -electrified wire. With this 

 object very finely-divided water-spray strongly negatively 

 electrified was projected into the room ; the spray was pro- 

 duced by forcing a jet of water under high pressure through 

 a small hole in a negatively-electrified plate ; the spray as it 

 fell was collected on filter-paper which, when drenched by the 

 spray, was placed in the tank through which the saturation- 

 current was measured. Very large effects were obtained in 

 this way, so large, indeed, that it seemed unlikely that they 

 were due to induced radio-activity on the negatively-electrified 

 spray. A series of experiments were accordingly made to see 

 whether wet paper produced any effect when the water had not 

 previously been electrified. Strips of wet filter-paper were 

 wrapped round the rods used as electrodes in the preceding 

 experiments, and much larger saturation-currents were ob- 

 tained with these electrodes than with bare rods. The current 

 was found to vary much with the nature of the paper — 

 filter, blotting, tissue, cartridge, and foolscap papers were 

 tried — and also with the nature of the solution; the results 

 obtained were complex and irregular, and this method of 



