1898.] and the Ionization produced in it by Rontgen Rays. 11 



far as possible the effects of variations in the conditions of the 

 bulb. 



This was done by having two vessels A and B exposed to the 

 rays simultaneously. B was used as a standard and was always 

 filled with air while A was filled alternately with air and the gas 

 under examination. Both A and B contained electrodes and the 

 current of electricity between these electrodes was measured. 



At first A and B were both filled with air and a series of 

 readings of the leaks through A and B taken alternately until 

 the constancy of the reading showed that the radiation from the 

 bulb was approximately steady. These readings gave the ratio 

 of the saturation currents through A and B when both were 

 filled with air. A was now filled with the gas to be examined 

 and the saturation currents through A and B again determined. 

 These readings gave the ratio of the saturation current through 

 A when filled with the gas to the saturation current through B 

 when filled with air. From the first observation however we 

 know the ratio of the saturation current through A when filled 

 with air to that through B when filled with air. Combining these 

 results we get the ratio of the saturation current through A when 

 filled with the gas to the saturation current through A when 

 filled with air. This ratio is the ratio of the ionization of the 

 gas to the ionization of air. 



The current in A and B passed between parallel plates fur- 

 nished with a guard ring as in the figure. The plates were placed 



so that the Rontgen rays passed through the gas in a direction 

 parallel to the plates. This arrangement was adopted to avoid 

 the secondary effects which as Perrin has shown arise when the 

 Rontgen rays strike against a metallic surface. The vessel B 

 which was always filled with air was left open so that the rays 

 went directly through the gas between the plates, the other vessel 

 A which had to be filled with different gases was provided with 

 an aluminium window through which the rays passed into the 

 vessel. All the insulating supports used to support the plates 

 in their position had to be screened from the Rontgen rays by 

 metal screens; every solid against which the rays struck was 

 made of metal, as the effects were found to be very irregular 

 when this precaution was neglected. The coil and the exhausted 

 tube used to produce the rays were in a large iron tank in which 

 two aluminium windows were inserted, the vessels A and B were 

 placed in front of these windows. 



