THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] 



FEBRUARY 1905. 



XIX. The Cathode Fall from Hot Carbon. By Professor 

 J. A. Cunningham, B.A. [R. U.I. $ Cambs.) ; A.R.C.Sc.I., 

 Presidency College. Calcutta : formerly 1851 Exhibition 

 Research Scholar. Cavendish Laboratory, Cambridge*. 



TI^HE experiments described in the present paper were 

 1 undertaken in continuation of the work already com- 

 municated to this Magazine (Dec. 1902, p. 6$±). For that 

 work a platinum-wire grating through which a heating 

 current could be passed was used as electrode. But it was 

 found that platinum became soft, and finally fused just at a 

 critically interesting >tage in the transformations of the 

 discharge. It was therefore natural to turn to carbon as 

 the most infusible material. This required the exclusion of 

 oxygen from the gas employed. It entailed, of course, also 

 considerably greater difficulty in the making of the apparatus. 

 The discharge-tnbe consisted of a stout glass tube 60 centi- 

 metres long and 25 cms. internal diameter, kept in a horizontal 

 position. Near the middle of this there were attached two side- 

 tubes which served to hold the fixed electrodes (A and K) at a 

 distance of 10*1 cms. apart along the axis of the discharge-tube 

 [fig. ]). A third narrow tube made connexion with the Topler 

 pump, McLeod gauge, and a three-way tap, which served for 

 tin* introduction of fresh supplies of gas and to cut off connexion 

 with the mercury of the pump when not in use, so as to 

 minimise the diffusion of mercury vapour into the discharge- 

 tube. Wound the cathode the tube was expanded to a 

 diameter of 3*8 cms. internally. The central portion of the 

 ■■■'- Communicated by Prof. J. J. Thomson. 

 Mag. S. 6. Vol. !>. No. 50. Feb. 1905. 



