344 HERMANN VON HELMHOLTZ 



In 1883 the Prussian Board of Education, at the request of 

 Helmholtz, invited his Assistant, Heinrich Hertz, to receive the 

 degree of Dozent in view of his approaching call to Kiel ; and 

 he took over from Helmholtz, whose activities were now 

 devoted entirely to other fields, the task of further exploring 

 the difficult and still unsolved problems of the doctrine of 

 electricity, on the principles of the Faraday- Maxwell hypothesis. 



Hertz had begun an investigation in Helmholtz's Institute 

 at Berlin which he concluded in this same year at Kiel, and 

 published as ' Experiments on the Glow Discharge '. It was con- 

 cerned with the form of discharge occurring in vacuous vessels, 

 which is accompanied by the phenomena of the cathode rays, 

 and of the striated positive light. Hertz finds that the cathode 

 rays do not deflect a magnetic needle, and so do not produce the 

 electrodynamic effect of a current; and he therefore regards 

 them as being only the accompaniment of a current, and not 

 as themselves constituting a current. 



On July 29, 1883, Helmholtz writes to Hertz: 



' I have read your investigation on the Glow Discharge with 

 the greatest interest, and cannot refrain from writing to say 

 Bravo ! The subject seems to me to be of very wide importance. 

 I have been considering for some time whether the cathode 

 rays may not be a mode of propagation of a sudden impact 

 upon the Maxwellian electromagnetic ether, in which the 

 surface of the electrodes forms the first wave-surface. For, as 

 far as I can see, such a wave should be propagated just as these 

 rays are. Deviation of the rays through magnetization of the 

 medium would accordingly also be possible ; longitudinal waves 

 could be more easily conceived, and might exist if the constant 

 k in my electromagnetic researches were not zero. But in 

 that case, transversal waves could also be produced. You 

 seem to have the same idea, but, however that may be, do not 

 hesitate to make use of my suggestion, for I have no time 

 at present to work it out. Besides, these thoughts arise 

 so readily in reading your investigation, that they would be 

 bound to occur to you soon, if they have not done so already. 

 One objection to your experiments, however, occurs to me 

 which you may perhaps be able to remove better than I, and 

 which in any case must be mentioned. This is that if the 

 cathode rays are electrical currents, according to the earlier 



