PRODUCED IN GASES BY RONTGEN RAYS. 
205 
reaching D. As for each reading this effect starts from zero, the only influence of 
this upon a series of readings with different potentials is to diminish their values by 
small amounts nearly proportional to their size, thus having practically no effect upon 
the result obtained by projecting the curve as in fig. 4. 
13. The velocity of the ions is evidently dependent upon the pressure of the gas. 
In these experiments the variations in the pressure were but small, being due mainly 
to the variations of the barometer. No experiments have been carried out on the 
effect of pressure upon the velocity of the ions produced by Rontgen rays, but 
E. Rutherford* has shown that for the conduction produced by ultra-violet light 
the velocities of the ions in air are inversely as the pressure of the gas. This result 
will be used in these experiments to reduce all of the values of the velocities to the 
same pressure of 7 6 centims. of mercury. 
14. The effect of temperature upon the ionic velocity is not known, so that correc¬ 
tions for temperature could not be made. The temperature was. however, taken in 
all cases, so that if necessary the correction can be applied later on. 
15. In considering the various corrections above, it is seen that the effect of many 
of them is diminished or made negligible by using a narrow beam of weak rays, and 
by using unpolished aluminium for that part of the cylinders where the rays impinge. 
Those corrections which depend upon the time required for the ions to cross between 
the two cylinders could be made very small by sufficiently reducing the value of this 
time, but we are limited in doing so by the increase that is produced in the difficulty 
of measuring one of the required quantities. Resort must be had to finding the 
values of the ionic velocities for different times of crossing, and from these deriving 
the final results. 
An estimated correction of 2 per cent, will be made for those effects considered 
above, especially (7) and (9), wdricli tend to make the result too small by an undeter¬ 
mined but small amount. 
§ 5. Changes made in Experimental Conditions. 
The apparatus used permits of several changes in the experimental conditions, 
which are a test of the accuracy of the method, and allow us to draw conclusions 
about the effects of some of the corrections previously noted. 
1. The velocity of the gas stream was varied by changing the weights on the 
gasometer. This necessitated a proportionate change in the value of the potential A 
of equation (9). The paths described by the ions are the same, but the time required 
for their passage between the two cylinders is changed. There are also changes in the 
amount of recombination of the ions and in the diffusion effect. The density of the 
free charges is changed, and so their effect upon the electric intensity is altered, and 
the spreading due to the mutual repulsion of the ions is also different. 
* E. Rutherford, ‘Proc. Camb. Phil. Soc.,’ vol. 9, Pt. VIII., p. tit. 
