552 Prof. J. J. Thomson on the Masses of 



ought to be the same : the small increase (amounting to not 

 more than 3 or 4 per cent.) may be due to the obliquity of 

 the path of the particles in the magnetic field, causing more 

 of them to be caught by the wires of the grating than would 

 be the case if the paths of the particles were at right angles 

 to the plane of the gauze. When the difference of potential 

 is reduced below a certain value, the deflexion of the elec- 

 trometer is very much reduced by the magnetic field ; it is 

 not, however, at once entirely destroyed when the potential- 

 difference passes through the critical value. The simple 

 theory just given would indicate a very abrupt transition from 

 the case when the magnetic force produces no effect, to that 

 in which it entirely stops the flow of negative electricity to 

 CD. In practice, however, I find that the transition is not 

 abrupt : after passing a certain difference of potential the 

 diminution in the electric charge received by CD increases 

 gradually as the potential-difference is reduced, and there is 

 not an abrupt transition from zero effect to a complete 

 stoppage of the leak between AB and CD. I think this is 

 due to the ionization not being confined to the gas in con- 

 tact with the illuminated plate, but extending through a 

 layer of gas whose thickness at very low pressures is quite 

 appreciable. The existence of a layer of this kind is indi- 

 cated by an experiment of Stoletow's, Stoletow found 

 that the maximum current between two plates depended at 

 low r pressures to a considerable extent upon the distance 

 between the plates, increasing as the distance between the 

 plates was increased. Now the maximum current is the one 

 that in one second uses up as many ions as are produced in 

 that time by the ultra-violet light. If all the ions are produced 

 close to the illuminated plate, increasing the distance between 

 the plates will not increase the number of ions available for 

 carrying the current ; if, however, the ions are produced in a 

 layer of sensible thickness, then, until the distance between 

 the plates exceeds the thickness of this layer, an increase in 

 the distance between the plates will increase the number of 

 ions, and so increase the maximum current. If this layer 

 has a sensible thickness, then the distance d which has to 

 be traversed by the ions before reaching the gauze connected 

 with the electrometer ranges from the distance between the 

 plates to the difference between this distance and the thick- 

 ness of the layer. The first ions to be stopped by the 

 magnetic field will be those coming from the surface of the 

 illuminated plate, as for these d has the greatest value : hence 

 we may use the equation , Y 



d =^ (D 



