loNIZATION BY NEGATIVE IONS 11 



5. Conductivity produced by Rontgen rays in a gas between 

 cylindrical electrodes. 



There is one point in connection with the current- 

 electric force curves obtained with Kontgen rays which 

 remains to be decided by experiment. In this case both 

 positive and negative ions are produced initially in the 

 gas by the rays, so that it is not evident from the 

 experiments with parallel plates that the results are due 

 to negative ions. Obviously the effects might be due to 

 positive ions if it happened that they had the property 

 of generating others by collisions while the negative 

 ions were inactive. It is, however, easy to show by using 

 electrodes of different shapes that the effects must be 

 attributed to negative ions. 1 If the gas is contained in 

 a spherical conductor which acts as one of the electrodes, 

 the other electrode being a small sphere at the centre, 

 or if a cylinder and a co-axial wire of small radius are 

 used as electrodes, then for small differences of potential 

 the current through the gas is the same in both directions 

 and corresponds to the total number of ions generated 

 by the rays; but when the difference of potential is 

 large the currents are no longer equal. It has been 

 found that when the outer electrode is negative and the 

 electromotive force is increased a large increase in the 

 current is obtained. In this case all the negative ions 

 produced by the rays traverse the field of strong electric 

 force in the neighbourhood of the small inner electrode, 

 and acquire sufficient velocity to generate others by 

 collisions with molecules of the gas. On the other 

 hand, when the outer electrode is positive and similar 



1 See Nature, Vol. Ixii., August 9, 1900. 



