CHAPTER V. 

 THE MERCURY ARC. 



THE mercury arc is an arc in a vacuum having mercury 

 for the cathode and either mercury or any other substance 

 which does not form an amalgam with it for the anode. 

 Tubes containing such arcs are usually called mercury 

 vapor lamps. This lamp was first studied by Arons in 

 1 892* and was developed by Cooper Hewitt into a valu- 

 able commercial form in 1902? The commercial use has 

 added much interest to the study of this arc and it is 

 also by far the best form for scientific purposes. One can 

 not, for example, begin to explain any of the phenomena 

 of the arc without having some idea of the cause of its 

 electrical conductivity. This requires some knowledge of 

 the temperature of the vapor and of the electric force 

 through this vapor, and the mercury arc offers the best 

 chance for accurate measurements of such quantities. 

 With this there are no complications due to chemical 

 action and the temperature is such that detailed measure- 

 ments can here be made with fine wires which do not 

 seriously change the character of the discharge. For this 

 reason this arc is described more fully than any other in the 

 following pages. 



For commercial use the mercury arc has several impor- 

 tant advantages. The first of these is that the light is not 



1 Wied. Ann., 47, 767; 1892. 



2 Trans. Amer. Elec. Inst., 19, 59, and 20, 929; 1902. 



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