lONIZATION BY NEGATIVE IONS 



4. Determination of a from experiments with Rbntgen rays. 



When the initial ionization is not produced at an 

 electrode, but consists of positive and negative ions 

 generated in the gas between the electrodes, as, for 

 instance, by Bontgen rays or by Becquerel rays, the effect 

 produced by collisions of the negative ions may be 

 calculated as follows : Let each negative ion produce a 

 others by collisions whilst passing through a centimetre 

 of the gas, and let n Q be the total number produced by 

 the rays between the plates when at a distance I apart. 

 In the layer of thickness dx at a distance x from the 



positive electrode a number , are generated by the 

 rays, and in passing through the distance x this number 



M fi <y* ft T 



is increased to -A Xe so that the total number of 

 negative ions n reaching the positive electrode is 



al 



77 



The ratio may be obtained from the curve giving 



HQ 



the connection between the current and the electric 

 force, TCQ being the constant current when no new ions 

 are produced by collisions, and n the current corre- 

 sponding to a force X. The value of a for the force X 



al -l 

 is therefore given by the equation =- =. The 



HQ Cll 



accuracy of this formula has been tested experimentally, 

 and it has been found that for a given force the value of 



varies with the distance I, but that the values of a 

 no 



