362 Prof. J. J. Thomson on easily Absorbed 



fastened on the brass plate: inside this tube there was an 

 insulated handle carrying the metal disk C, which was con- 

 nected with one pair of the quadrants of an electrometer. 

 The metal tube and the brass end of the discharge-tube were 

 kept permanently connected with the earth ; the other elec- 

 trode of the discharge-tube was an aluminium disk. To test 

 whether any radiation passed through the windows in the 

 brass plate the disk C was charged up, and — the insulation 

 having been found to be practically perfect when no discharge 

 was passing through the tube — the discharge was turned on; 

 it was then found that under certain circumstances the 

 electricity leaked from C. This was the method most fre- 

 quently used, but in other cases the presence of radiation was 

 tested by taking photographs outside the tube behind the 

 windows. In the earlier experiments, the discharge was 

 produced by an induction-coil giving about 25 cm. sparks, 

 and it was found that even when the pressure in the tube was 

 so high that the dark space was not more than 3 millimetres 

 thick, the disk C began to leak perceptibly when the discharge 

 passed through the tube. It leaked more quickly when the 

 brass plate was the negative than when it was the positive 

 electrode. The potential-difference between the electrodes 

 was, however, less in the second case than in the first, as the 

 mica in front of the brass plate made the effective area of this 

 electrode less than that of the electrode B, and when the 

 discharge passes between electrodes of different sizes the 

 potential-difference between the electrodes is less when the 

 larger electrode is cathode than when it is anode. 



The rate of leak from the disk C was found to be independent 

 of the sign of its electrification : and the disk did not receive a 

 charge if it were uncharged to begin with. The leak shows 

 that the gas outside the tube is ionized, and that this ioniza- 

 tion is similar to that produced by Rontgen rays. To see 

 whether the effects outside the tube corresponded in other 

 respects to those produced by Rontgen rays, a photographic 

 plate was placed outside the tube, behind the holes, and with 

 an exposure of about 5 minutes very good photographs of the 

 holes were obtained, the photographs being denser when the 

 brass disk was cathode than when it was anode. There is 

 thus outside the tube even when the gas is at a pressure very 

 much greater than that in ordinary Rontgen tubes a radiation 

 which possesses properties similar to the Rontgen radiation, 



As the potential-difference required to produce the dis- 

 charge at these high pressures is extremely small, it was 

 thought desirable to replace the induction-coil by a battery 

 of small storage-cells. With these cells it was found possible 



