[MCLENNAN] TEMPERATURE OF THE MERCURY ARC 65 



One point which is emphasized by the measurements is the marked 

 difference between the temperature of the liquid mercury at the cathode 

 and that of the mercury vapour in the arc. The former it will be 

 seen reached only 158-2° C, while the latter rose as high as 326° C. 



The anode in the mercury tube was 60 cms. from the closed end 

 of the quartz tube and the luminosity in the arc extended through- 

 out the whole of the vapour from the mercury cathode up to the iron 

 anode. It will be seen from the curve that while a maximum tempera- 

 ture was reached at a point about 15 cm. from the end of the tube, 

 the temperature was about the same over the whole length of the 

 arc and varied only from about 305 to 326 degrees. On the other 

 hand the curve shews that the temperature gradient was very steep 

 in the neighbourhood I^oth of the cathode and of the anode. 



While these numbers give the temperature to which the thermo- 

 couple was subjected at different points along the tube it is not to be 



Figure 4 



understood that they represent even approximately the actual tem- 

 perature of the luminous mercury vapour in the arc. Geiger* who 

 has studied the temperatures in electrical discharges in hydrogen, 

 nitrogen and atmospheric air at low pressures in Geissler tubes pro- 

 vided with Wehnelt cathodes has shewn that with currents of about 

 an ampere passing in the tubes temperatures beyond 1000° C have 

 been observed with thermocouples directly exposed to the discharge. 

 These high temperatures were also found to characterise the 

 discharge in the mercury arc when a thermocouple was exposed 

 directly to the luminous vapour. To bring out this point a tube of 

 the form shewn in Fig. 4 was used and measurements were made on 

 the discharge in it with a platinum platinum-iridium thermo-couple, 

 b, d, e, sealed into it with one junction, b, situated at the axis of the 

 tube. The terminals d and e were joined directly to the compen- 

 sation apparatus referred to above and this gave the electromotive 

 forces of the junction when discharges of different intensities were 

 sent through the tube. 



*Geiger, Inaug. Dissert., Erlangen, July, 1906. 



