[mclennan-dearle] spectrum OF THE MERCURY ARC 



181 



showed that it consisted of two wave-lengths, the one at about 218fi 

 and the other in the neighbourhood of 343/x. 



The only noteworthy measurements by photographic methods 

 in the infra-red spectrum of the mercury arc were made in 1912 by 

 H. Lehmann,^ He used the phosphoro-photographic method of 

 Bergmann^ and found four lines corresponding to those of Paschen 

 and in addition a new line at 1 -46/1. 



In the present investigation a careful survey was made of the 

 infra-red spectrum of the mercury arc in the region beyond 1 • Oju 

 with the object of confirming, if possible, the existence of the lines 

 identified by Paschen and of seeing whether the lines found by Cob- 

 lentz and Geer in the neighbourhood of 3fi, had a real existence. In 

 this examination a number of the lines noted by Paschen were identi- 

 fied, the existence of a line near 3-On was confirmed and in addition 

 a number of new lines were observed. 



II. Apparatus. 



In this work a number of different forms of mercury arc lamps 

 were used as sources of the radiation; but a quartz mercury lamp, 

 constructed by W. C. Heraeus, was found to be the most satisfactory. 

 This lamp gave a very powerful and quite concentrated arc. When in 

 operation it was driven with a direct current of from 3-0 to 3-2 

 amperes, with a striking potential of 110 volts. A suitable resistance 

 was of course inserted in series with the lamp. It was found that even 

 with this lamp the current through it steadily decreased for the first 

 |ten minutes after the arc was struck. This was brought about by an 

 increase which took place in the resistance of the mercury vapour 

 when the temperature of the lamp was rising. The following table 

 and the curve in Fig. 1. show the variation of the current with the 

 elapse of time in a typical case: 



iH. Lehmann. Ann. der. Phys. 39, 1, pp. 76-77, 1912, 



^Bergmann, Zeitschr. Wiss. Phot. 6 pp. 113-130 and pp. 145-169, 1908. 



