on the Ionization produced in Gases by Rontgen Rays. 195 



the rays given out is apt to vary slightly during a series of 

 observations extending over several hours, and it was therefore 

 deemed advisable to have some sort of test of the intensity of the 

 rays, apart from the ionization in the cylinder, since the condition 

 of the gas with regard to temperature was to change in the 

 cylinder during the course of the experiment. For this purpose 

 a small test apparatus was introduced in the path of the rays 

 between the bulb and the cylinder, as shown in the diagram. This 

 consisted simply of a short rectangular cylinder made of sheet 

 lead, the ends of which were closed with ordinary white paper. 

 Inside this were placed two insulated parallel plate electrodes, 

 which consisted of exceedingly thin aluminium foil, stretched on 

 metal frames for support. The rays easily passed through the 

 thin aluminium foils and ionized the air between them and the 

 measure of the ionization between the plates was a test of the 

 intensity of the rays from time to time, for any change in intensity 

 would of course be indicated by a corresponding change in the 

 ionization produced. One of these plates was connected to the 

 same pole of the battery as the electrode E' while the other plate 

 was connected, in parallel with a condenser G through the key W, 

 to the same pair of quadrants of the electrometer as the elec- 

 trode E. 



Method of observations. 



In measuring the ionization the observations were taken in the 

 following manner. Both pairs of quadrants of the electrometer 

 and the electrode E were connected to earth and also the electrode 

 / was earthed. The Rontgen ray bulb was started and allowed 

 to run for twenty seconds so that the ionization in the cylinder 

 AB might reach a steady state. At the end of the twenty seconds 

 the one pair of quadrants of the electrometer along with the 

 electrode E was insulated. The electrode / was also insulated and 

 disconnected from the electrometer and the electrode E by opening 

 the key W at the same instant that E was insulated. Both the 

 key W and the one insulating E were worked at a distance by 

 means of cords. After insulating these electrodes the bulb was 

 allowed to run for five seconds, at the end of which it was shut off. 

 Both the electrodes E and / were thus allowed to charge up for 

 exactly the same time, and the gas in each case was ionized by the 

 same cone of rays and consequently any change in intensity in one 

 case would produce a corresponding change in the other. The air 

 in the standard apparatus was thus always ionized at precisely the 

 same time as the gas in the testing cylinder, and since the ioniza- 

 tion in the standard was independent of what was going on in the 

 testing cylinder it formed a check on the intensity of the rays 



