580 Mr. A. E. Grarrett on Positive Electrification 



pressure attains the value 5 or 6 cms., after this the decrease 

 which takes place in the current for any further increase in 

 the pressure is always smaller but is still quite marked. The 

 rate of decrease is always more rapid at the higher tempera- 

 tures. 



When the pressures for which the currents have maximum 

 values are plotted against the temperatures, it appears as if 

 the pressure and temperature are connected by a straight-line 

 law, and that if the temperature could be pushed to a 

 sufficiently high degree the largest current would be obtained 

 in the highest attainable vacuum. 



The fact that the maximum currents obtained at the higher 

 temperatures and lower pressures were always greater than 

 those obtained with the lower temperatures and higher 

 pressures is further evidence in support of this view. 



It is quite possible, however, that the slope of the tempera- 

 ture-pressure line may gradually become less and less, and 

 that it never actually reaches the zero pressure line. 



The peculiar manner in which the current varies with the 

 pressure under these conditions cannot be explained by the 

 collision of moving ions, as in all cases the voltage used was 

 that corresponding to the flat part of the saturation curve. 

 Evidence given later suggests that neutral doublets, such as 

 Righi suggested compose the magneto-cathode rays, and Sir 

 J. J. Thomson * found indications of in his experiments on 

 positive electricity, are driven off when the salt is heated, and 

 the current may be due to the break up of these doublets by 

 collision with the gas molecules. 



These doublets would be shot off with relatively large 

 velocities at the lower pressures, but would have small chances 

 of colliding with gas molecules or other doublets, and so 

 relatively few free ions are produced. At higher pressures 

 the doublets would have much slower speeds but greater 

 chances of collision. Somewhere between these extremes the 

 most favourable conditions for obtaining a maximum current 

 may be looked for at each temperature. 



At the higher temperatures the velocity of ejection is 

 greater, and a less number of collisions will be required in 

 order to split up the doublet, hence the maximum current is 

 obtained under such conditions at a lower pressure. 



With an increase of pressure more collisions are possible,, 

 but less doublets escape from the heated salt since the 

 temperature is lower, hence the current obtained is smaller. 



The great drawback in the way of this explanation is the 



* Phil. Mag. xviii. pp. 828 et seq., Dec. 1909. 



