lONIZATION BY NEGATIVE IONS 31 



from which these values could be deduced. Several 

 experimental investigations, however, have been made 

 by other physicists of the conductivity produced by 

 external agencies in gases at low pressures. Among 

 the earliest were those by Stoletow, 1 who showed 

 that there are large variations in the currents 

 between parallel plates due to changes in the force 

 and pressure when ultra-violet light acts on the negative 

 electrode. 



Most of the effects which were observed may obviously 

 be attributed to the development of large currents by 

 the collisions of the ions with the molecules of the gas, 

 and a detailed examination of all these experiments is 

 unnecessary. One result, however, obtained by Stoletow, 

 may be mentioned, as it requires some consideration to 

 see how it can be explained by the theory. 



It was found that when the distance and potential fall 

 between the electrodes is constant, the current increases 

 as the pressure of the gas is diminished, that at a certain 

 pressure P the current attains a maximum value, and 

 that when the pressure is reduced below the value P the 

 current diminishes. The pressure P, at which the 

 current attains the maximum value, is proportional to 

 the force X and independent of the distance between the 

 plates. 



The following table is given by Stoletow for air, 2 E being 

 the electromotive force between the two plates, the unit 

 being the electromotive force of a Clark cell, I the distance 

 between the plates in millimetres, and P the pressure, 



1 Stoletow, Comptes JRendus, t. cvii., p. 91, 1888; Journal de 

 Physique, Serie ii., t. ix., 1890. 



3 Stoletow, Journal de Physique, loc. cit. 



