PRODUCED IN GASES BY RONTGEN RAYS. 
215 
in the ratio of three to one, but no noticeable change in the result could be observed. 
During the course of all of the experiments the rays were not of the same intensity, 
for the Crookes’ tube had to be replaced several times, but in all cases without any 
marked effect upon the values obtained. It must be said, however, that rays of great 
intensity were never employed, the aim being always to have them as weak as 
possible for reasons previously stated. 
The most severe test to which the method was subjected was a change in the 
dimensions of the inner cylinder. In the above experiments the diameter of this 
cylinder was 1 centim., and it was now exchanged for one having a diameter of 
2 - 8 centims. The distance between the inner and the outer cylinders was thus 
diminished to nearly one-half of its former value. The electric field between the two 
became much more uniform, and the gas velocities for different points of a cross- 
section now varied in a different manner. In order to keep the other quantities the 
same, the small distance between the two cylinders necessitated the use of voltages 
only about one-quarter as large as those used in the former arrangement. This 
increased the difficulty of the measurements and also some of the corrections which 
must be applied to get the final result. The density of the free charges in the gas 
was greater because the ions moved slower, being in a weaker field, and the same fall 
'of potential at the electrodes was a larger percentage of the total voltage. The 
width of the beam of rays used was ‘3 centim. 
The following is a summary of the results obtained :— 
