100 THE ELECTRIC ARC 



In some work by myself l it was shown that the front of 

 this luminous region where the mercury is condensing to 

 the liquid form has the greatest conductivity and the 

 greatest luminosity. It was also shown that this conduc- 

 tivity is not due to any rays sent out from the arc itself. 



Mercury Arc in Quartz Tubes. The light efficiency of 

 mercury vapor lamps is higher with large currents, but 

 tubes made of ordinary glass will melt when such currents 

 are used. To avoid this difficulty lamps are now being 

 made of fused quartz. These lamps are among the very 

 best as far as watts per candle power are concerned. They 

 also give off a great amount of ultra-violet light, which is 

 advantageous for photographic work, and because of these 

 rays they can be used for sterilizing water. 



The light from such lamps appears to the eye whiter 

 than that from the common mercury vapor lamps and yet it 

 is lacking in certain important parts of the spectrum and 

 has the added disadvantage that the ultra-violet rays are 

 very injurious to the eyes. However, these rays may be 

 screened off by common glass, there being sufficient pro- 

 tection, if the quartz lamp is placed inside a glass globe. 

 It is also claimed that the rays are absorbed by a few feet 

 of air, so that lamps placed fifteen or twenty feet above a 

 sidewalk would not be injurious to passers-by, even if the 

 glass globe should be broken. 



The relation between current, potential difference and 

 the pressure of the mercury vapor in such lamps has been 

 examined by Kuch and Retschinsky. 2 As the tube was 

 allowed to become hot the vapor pressure rose and a larger 

 potential difference was necessary in order to maintain 



1 Phys. Rev., 22, 221; 1906. 



2 Ann. d. Phys., 20, 563; 1906. 



