THE MERCURY ARC 103 



ing when a zinc amalgam was placed in the tube or pieces 

 of aluminum were allowed to float on the mercury. With 

 both tin and aluminum amalgams the potential difference, 

 was much higher than with pure mercury. 



More successful results were attained by Gehrke and 

 Baeyer 1 who found that an amalgam of 100 parts, by weight, 

 of zinc and 30 parts of mercury gives a much better light 

 than mercury alone. Still better results have been secured 

 when using amalgams in quartz tubes, since these tubes 

 can stand a higher temperature and are not acted on chemi- 

 cally. Arons 2 recommends trie following for use in quartz 

 tubes: mercury 59 per cent, lead 20 per cent, bismuth 

 20 per cent, zinc 0.5 per cent and cadmium 0.5 per cent. 



Quite a different method of supplying the deficient light 

 has been suggested by Cooper Hewitt. 3 This is to use a. 

 reflector on whose surface are crystals which emit red 

 light when illuminated by ultra-violet rays. No data have 

 as yet been published concerning the amount of red light 

 which is thus obtained and it does not seem to have met 

 with any commercial success. (. 



Another device and one which promises excellent results 

 is a lamp described by Urbain, Seal and Feige. 4 In this 

 the anode is tungsten and the density of the current is such 

 that this is raised to a very high temperature and gives out 

 the greater part of the light. This must be in a vacuum 

 or in an inert gas, since a trace of oxygen is sufficient to 

 blacken the surrounding globe. The principal use of the 

 mercury is to start the arc, it being difficult to start an arc 



1 Elektrot. ZS., 27, 383; 1906. 



2 Lond. Elec., 62, 387; 1908. 



3 Elec. World, 56, 1343; 1910. 



4 L'Industrie Elec., March 10, 1911. 



