196 Professor Joseph John Thomson [Jan. 19, 



energy that its equilibrium l^ecomes unstable. Other phenomena 

 point to this as the method by which ionization is effected. If the cor- 

 puscles are dragged out of the atoms by the electric field, the velocity 

 with which they are projected should depend upon the strength of 

 the field ; while, if they are projected by an explosion, their velocity 

 would depend upon the nature of the atom and not upon the strength 

 of the field. Now when Rontgen rays fall upon a substance, the 

 atoms of the substance are ionized, and corpuscles, forming a stream 

 of cathodic rays, are emitted. Barkla has lately shown, however, 

 that the penetrating power of the cathodic rays produced in this 

 way is independent of the intensity of the Rontgen rays. Now the 

 electric force in the Rontgen rays depends upon their intensity, and 

 the penetrating power of the cathodic rays depends upon their 

 velocity ; so that this result shows that the velocity of the corpuscles 

 does not depend upon the intensity of the force acting upon them. 

 Again, Lenard has shown that the velocity of the corpuscles ejected 

 when ultra-violet light falls upon a metal is independent of the 

 intensity of the light. Lenard also investigated the secondary 

 cathode rays produced when cathode rays fall upon matter, and found 

 that, in addition to rays, whose velocity was of the same order as that 

 of the primary rays , and which may be regarded as deflected primary 

 rays, there were other very slow rays ; and the measurements he gives 

 indicate that the velocity of these varies but little with that of the 

 primary rays. 



A point of great importance, which can easily be shown by this 

 apparatus, is that the stage at which luminosity sets in depends upon 

 the current density through the tube, and not merely upon the poten- 

 tial difference. One way of showing this is to lower the temperature 

 of the platinum, keeping all the other conditions the same, and again 

 determine the relation between the current and the potential differ- 

 ence. The effect of lowering the temperature is to reduce the number 

 of corpuscles starting from the cathode, so that with the same poten- 

 tial difference the cuiTent density is smaller. If the relation between 

 the current and potential difference are represented by curves, such as 

 those in Fig. 3, it will be seen at once that the curve for the cooler 

 electrode cannot be deduced from that for the hotter by reducing all 

 the ordinates in the same proportion. The critical points on the 

 curves, i.e. the place where ionization by collision begins and where 

 the luminous discharge appears, are at very different potentials : the 

 greater the current density the smaller the potential difference 

 corresponding to these critical points. Thus, to take a case actually 

 o])scrvcd, when the wire was very hot the discharge was brightly 

 luminous with a potential of 24 volts, but on lowering the tempera- 

 ture no luminosity could be detected with a potential difference of 

 110 volts. 



We can also show the effect of current density without altering 

 the temperature of the cathode, by placing near the tube an electro- 



