Conducting Gases on the Direction of the Electric Field. 211 



the case of potential -differences of the order of one volt*, 

 when the contact-differences o£ potential between the elec- 

 trodes may cause inequalities in the currents. In the general 

 case, however, of gas-conduction, I have found that there is, 

 in most cases, a difference between the values of the current 

 with the reversal of the electric field. This difference is due 

 to the inequality of the velocities of the positive and negative 

 ions. 



It is only in certain special cases that the current is 

 independent of the direction of the electric field. 



These cases are : — 



(1) When the ionization of the gas is symmetrical with 



regard to the electrodes. 



(2) When the electric field is so great that the current is a 

 maximum, i.e. when all the ions reach the electrodes 

 before recombination occurs. 



(3) When the number of ions present is so small that their 

 movement between the charged electrodes does not 

 appreciably disturb the potential-gradient. 



(4) When the positive and negative ions have equal 

 velocities. In all other cases the positive and negative 

 currents are unequal in value, the difference depending 

 upon the distribution and intensity of the ionization, the 

 distance apart and shape of the electrodes, and. the 

 potential-difference applied. 



In many of the experimental arrangements of previous 

 observers, by means of which the equality of the current in 

 the two directions was observed, one or more of the above 

 conditions was fulfilled. 



The essential conditions for obtaining unequal currents 

 are : — 



(1) Ionization unsymmetrical with regard to the electrodes. 



(2) Disturbance of the potential-gradient due to the move- 

 ment of the ions in the electric field. 



(3) Unequal velocity of the ions. 



A few simple cases will now be considered, in which the 

 difference between positive and negative currents is strongly 

 marked. 



The difference between the currents in air is most readily 

 shown when the gas is dry, and when the ionization between 

 the electrodes is powerful and confined mainly to the surface 

 of one electrode. This condition can readily be fulfilled by 

 allowing a thin stratum of strong Rontgen-rays to pass 

 between two parallel plate-electrodes, but nearer one plate 

 than the other, or by using a powerful radio-active substance, 

 * E. Rutherford, Phil. Mag. Jan. 1899. 

 P2 



