[ 614 ] 



LV. On the Dimensions of the Cathode Images. 

 By^. Hehl*. 



IN vol. iv. p. 608, of the Phil. Mag. for 1902, Mr. H. A. 

 Wilson gives an account of determinations of the current- 

 density at the cathodes of vacuum-tubes when the cathodes 

 are not completely covered with the glow. These determina- 

 tions lead to the same relation between current-density and 

 pressure as that which I had already discovered and discussed 

 in detail in 1901 in my Erlangen Dissertation f. 



In his experiments, Mr. H. A. Wilson made use of wire 

 cathodes which were placed along the axis of a tube 30 cms. 

 long and only 8 cms. wide. Such narrow tubes may, accord- 

 ing to my observations, lead to wrong results, by reason of 

 the fact that there is too little space to allow the glow to 

 spread freely. This accounts for the slight discrepancy 

 between our results, and induces me to publish a supple- 

 mentary account of the experimental arrangement employed 

 by myself, and of the results obtained in this and in other 

 directions. 



Description of Apparatus. 



The discharge-tubes employed consisted of large WulfFs 

 bottles 25 cms. high and 15 cms. in diameter, along whose 

 axes were arranged the cathodes to be investigated, which 

 consisted mostly of wire. The length of the image was 

 measured by means of an illuminated scale and telescope, 

 through which the luminous image and the cathode were 

 viewed simultaneously after reflexion in a totally reflecting 

 prism. The relations connecting the area of cathode covered 

 by the glow with the current and the pressure were deter- 

 mined. For full details, reference must be made to the 

 original paper J. 



Results Obtained. 



1. Relation connecting the area covered by the glow with the 

 current ivhen the pressure is\ constant. The measurements 

 showed that the length of ivire covered by the glow was directly 

 proportional to the current, or, since with a given wire the 

 lengths are proportional to the areas, that 



constant, (1) 



2wrl 



i being the current, r the radius, and / the length of the wire 



* Communication from the Physical Institute of the University of 

 Erlangen. Communicated by Prof. E. Wiedemann. 



f N. Hehl, Dissertation Erlangen, 1901 ; also Sitzungsberichte der 

 Phys.-Med. Societat Ertangen, 1901 ; and Phys. Zeitschr. iii. p. 547, 1902.. 



X Cf. Dissertation, p. 5. 



