92 THE ELECTRIC ARC 



One of these quantities is the temperature of the mercury 

 arc. Nothing would seem simpler than to insert a ther- 

 mometer or thermojunction in the mercury vapor and 

 observe its temperature. There are, indeed, several who 

 have made such measurements. Thus Arons measured this 

 temperature by means of a mercury thermometer and 

 found that it varied from 285 C. to 515 C. Wills 1 meas- 

 ured it with a thermojunction and found values varying 

 from 83 C. to 297 C., depending on the length of time 

 that the arc had been running and on the density of the 

 mercury vapor. Knipp also measured this temperature 

 with a thermojunction and showed that the temperature 

 increases rapidly as the pressure of the vapor increases. 2 

 However, both the thermometer and the thermojunction 

 give temperatures which are too low. Both radiate some 

 heat and conduct some away and must be cooler than the 

 vapor in order to receive heat from it. Arons, therefore, 

 made a further test by placing a fine platinum wire in the 

 vapor of the arc. He found that this was quickly melted. 

 There is, however, the possibility that the platinum wire 

 first formed an amalgam with the mercury and that this 

 caused the wire to melt at a comparatively low temperature. 

 I have, therefore, repeated the experiment using a glass 

 fiber and find that this also can be melted in the mercury 

 arc, but even here we are not on sure ground, for the glass 

 may receive its heat from the moving electrons and not 

 from the molecules of the vapor. 



Arons endeavored to compute the temperature of the 

 mercury vapor. We know approximately the amount 

 of gas in the tube, its specific heat and the rate at which 



1 Phys. Rev., 19, 65; 1904. 



2 Phys. Rev., 24, 446; 1907, and 31, 97; 1910. 



