PRODUCED IN GASES BY RONTGEN RAYS. 
203 
cylinders so that if we obtain values of the ionic velocity, in the manner already 
described, these will be the larger and nearer to the true value the smaller the 
time that is required for the passage of the ions across. 
If this time were zero, then evidently all diffusion effects would disappear. 
6. The free charges that exist in the gas, where the ions of one sign predominate, 
tend to spread on account of the mutual repulsion of the charged carriers. 
This produces an effect similar to that of the diffusion just described. It increases 
with the time required for the ions to pass between the cylinders, but is less the 
smaller the density of the free charges, i. e., the weaker the Rontgen rays used and 
the narrower the beam of the rays. 
7. The presence of these free charges in the gas also has an influence upon the 
intensity of the electrostatic field between the two cylinders. To diminish this effect 
a sensitive electrometer was used in making the observations, as this allowed the 
employment of a weak radiation so that the charges in the gas were of a small 
density. 
While it is not possible to make an exact calculation of the magnitude of this effect 
because of the unsymmetrical distribution of the ions, an approximation to it can still 
be obtained. Knowing the capacity of the receiving system and the charge received 
in a given time, and knowing the approximate velocity of the ions in the electric field 
and the approximate space occupied by the free charges, the density of these charges 
can be obtained roughly, and their effect upon the electrostatic field can be 
computed. 
Computations of this kind made from the observations used for final results 
showed that the largest value of this correction made a diminution in the electro¬ 
static field of less than 1 per cent. In some experiments where a large inner 
cylinder was used the intensity of the electric field employed was less, the ions 
moved slower, and the density of the free charges was therefore larger and in some 
instances the above correction was perhaps nearly 2 per cent. In all cases an 
increase in the strength of the field itself diminishes the percentage value of the 
correction, while the simultaneous diminution in the density of the free charges 
reduces it still further. 
8. The motion of these free charges through the gas also produces a motion of the 
gas itself, as the writer has previously shown. # The amount of this is, however, very 
small compared to the velocity of the ions, so that it cannot have an appreciable 
disturbing effect upon the results of these experiments. 
9. In conduction produced by Rontgen rays there is a noticeable fall of potential at 
the electrodes which diminishes the electric intensity in the intermediate space. As 
determined by the writer,! for conduction in air between two plates 1‘2 centims. 
apart, this amounted to about 2 per cent, of the total potential difference for the 
* J. Zeleny, ‘ Proc. Camb. Phil. Soc.,’ vol. 10, Pt. I., p. 13. 
T J. Zeleny, ‘ Proc. Camb. Phil. Soc.,’ vol. 10, Pt. I., p. 21. 
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