46 THE ELECTRIC ARC 



sudden changes in the current produced a decided effect 

 on the cathode. A sudden decrease in the current caused 

 the cathode to become dimmer, but it quickly became hot 

 again, although smaller in extent. 



An attempt was made by myself 1 to find the temperature 

 of the vapor of a carbon arc in a vacuum, but nothing 

 more definite was determined than that the temperature 

 with as low a pressure as 0.5 mm. was high enough to 

 melt a fine platinum wire. 2 



Effect Produced by Cooling the Electrodes. There has 

 been much discussion among experimenters as to whether 

 higher voltages are required with cooled electrodes or with 

 those not cooled. Since this has some bearing on the 

 theory of the arc it is a matter of importance. We may, 

 however, pass over the work of the earlier experimenters, 

 since they seem to have worked without due consideration 

 of the conditions. For example de la Rive 3 attempted to 

 heat the arc by placing an alcohol lamp under it. There 

 would be three results produced by this experiment other 

 than the one which he had in mind. First, the flame 

 would probably cool the carbons of the arc instead of 

 heating them. Secondly, the flame would blow the arc 

 out of its original shape, and thus cause a greater drop in 

 potential through it. Lastly, the gases of the flame would 

 change the character of the arc and this might have a 

 very appreciable effect on the voltage. 



Again Lecher 4 cooled one of the electrodes of the arc by 

 immersing it in a mercury bath which was in turn kept 



1 Phys. Rev., 19, 123; 1904. 



2 The temperature of the mercury arc will be considered in connection 

 with the other experiments on that arc. 



3 Archive de PElectricite, i, 262; 1841. 



4 Wien. Sitzungsber., 95, 2 A, 992; 1887. 



