[mclennan] 



TEMPERATURE OF THE MERCURY ARC 



63 



obtained in the ordinary way from the dealers, and in all probability 

 were not pure. They gave fairly high electromotive forces, however, 

 and consequently did very well for the use to which they were put. 



In taking the temperatures in the arc the one junction of the 

 couple was kept at 0° C with melting ice, and the other was placed at 

 different points in the quartz tube, commencing with the end of the 

 tube dipping into the mercury at the cathode. 



In each position the junction practically took up at once the 

 temperature in the tube at that point. The temperature at any given 

 point was found to increase with the consumption of energy passing in 



Zoo' SOO 



Figure 2 



Soo'C 



the arc, but when the energy expended in the latter was maintained 

 at a uniform rate the temperatures at the different points in the quartz 

 tube were found to remain quite steady. 



The results obtained in one set of observations are given in Table 

 II. When taking these readings a difference of potential of 45 volts 

 was maintained between the terminals of the lamp and the current 

 passing was 5 «4 amperes. In this table the numbers in column I 

 give the distance in centimetres from the closed end of the quartz 

 tube at which the heated junction was placed, and in column II are 

 given the electromotive forces of the couple obtained with the junction 

 at these various points. Column III gives the temperatures deduced from 

 the calibration curve which corresponded to these different electromotive 

 forces. These temperatures plotted against the distances of the points of 

 observation from the closed end of the quartz tube are shewn in Fig. 3. 



