1898.] Fall of Potential at the Electrodes, etc. 17 



All of the work now done by the field upon the moving ion is 

 transferred to the body of the resisting gas, which therefore tends 

 to move with the ion. 



If an equal number of positive and negative ions are moving 

 in opposite directions through a gas, the resulting velocity is nil ; 

 but if the ions of one sign are more numerous, that is, if a free 

 charge exists in the gas, it then tends to move with accelerated 

 motion in the direction taken by the predominant ions. 



Now in the case of conduction under the influence of Rontgen 

 rays it has been shown by J. J. Thomson and E. Rutherford* that 

 a free charge exists in the gas, and the writerf has investigated 

 the distribution of the charge between two plates and found that 

 near each plate there is a charge in the gas opposite in sign to 

 that of the plate, and that this decreases rapidly with the distance 

 from the plates. 



In the neighbourhood of each plate, therefore, there exists a 

 force tending to move the gas towards the plate, while near the 

 centre there is a neutral region. 



If the two plates were unlimited in extent, then the motion of 

 the gas would produce an inequality in the gas pressure which 

 would counteract all further movements ; but with limited plates 

 the excess pressure at their surfaces is relieved sideways, and 

 there results a set of convection currents whose velocity increases 

 until the loss of energy due to friction is equal to that gained 

 from the moving ions. 



In accordance with this view, in the experiments described 

 above it was seen that when the conduction begins the gas starts 

 to move towards the electrodes, that this motion is accelerated for 

 a noticeable length of time, that it is more vigorous with greater 

 ionization and greater potential gradients, and that finally it 

 assumes a steady state. 



The configuration of the convection currents in the apparatus 

 during the steady state has been traced in some cases, but is of no 

 especial interest as it is so dependent upon the form and size of 

 the apparatus used. 



The view taken of the nature of the convection currents has 

 been tested by a considerable number of experiments made with 

 different forms of apparatus and under varying conditions ; it may 

 be useful to mention here a simple modification of the experiments 

 already described. 



If in the apparatus shown in fig. 1 the rays instead of falling 

 upon all of the gas between the two plates are allowed to impinge 

 only on a part next to one plate, as may be done by screening the 

 other side by one of the movable strips of lead L, then as soon as 



* J. J. Thomson and E. Kutherford, Phil. Mag. Nov. 1896, p. 394. 

 t J. Zeleny, Phil. Mag. July 1898, p. 138. 



VOL. X. PT. I. 2 



