GENERATION, CONTROL, AND MEASUREMENT 



175 



at the lowest pressure the continuum is weak and most of the energy is 

 in the Hnes. 



The capillary arc is limited to about 1 kw for a capillary of 2 mm. 

 Since the energy is concentrated within the confines of the capillary, 

 it makes an excellent source for the irradiation of a monochromator slit. 



Short, or Concentrated, Arc. In order to obtain a source of more nearly 

 spherical dimensions and one not limited by the melting point of quartz, 



0.4 



3 



Q. 



0.2 



en 

 a 



< 



IE 



a. 



E 



0.4 



0.3 



2 



01 



300 400 500 600 700 300 400 500 600 700 

 4 1 ; 1 1 1 1 0.4 



03 



2 



< 01 



0.3 



02 



0.1 



300 400 



500 



500 



600 



700 



600 700 300 400 

 WAVE LENGTH, rriyU 



Fig. 3-14. Effect of mercury vapor pressure on the spectral energy distribution of 

 high-pressure capillary mercury arcs. Curve a, 54 atm; curve h, 102 atm; curve c, 

 197 atm; curve d, 319 atm. {From Forsythe, Adams, and Barnes, 1942.) 



the so-called "short-" or "concentrated-arc" lamp has been developed 

 (Anderson, 1951; Freeman, 1950). Massive tungsten electrodes, within 

 a large thick-walled spherical quartz envelope containing a small amount 

 of mercury, confine the arc to a short stream about 10 mm in length 

 and 5-10 mm in diameter. These lamps may be started automatically 

 with a high-voltage pulse circuit. Since the quartz envelope is some dis- 

 tance from the arc stream, it is not exposed to excessive temperatures. 

 This lamp is made in sizes up to 10 kw. The spectral characteristics of 

 the short-arc lamp are very similar to those of the high-pressure capillary 

 arc. 



Operation. Since liquid mercury at room temperature has too low a 

 vapor pressure to support a discharge at reasonable exciting potentials, 

 argon gas at several millimeters pressure is usually present. The dis- 

 charge is initiated in argon, and then, as the temperature rises and the 



