Emission Spectrum of the Mercury Arc. 687 



up to 10 fjb from data on the dispersion of rock-salt as given 

 by Paschen* and others. 



An eyepiece E was attachable behind the slit S 2 for the 

 purpose of focussing lines in the visible part of the spectrum 

 on the thermopile, and of adjusting the prism so that the 

 radiation brought to a focus at S 2 was in agreement with 

 the reading on the drum. The prism had faces 3'2 cm. by 

 4*2 cm., and was ground to an angle of approximately 55°. 

 Judging by the visible spectrum, there was very little curva- 

 ture in the spectral lines produced by this prism. 



The thermopile (fig. 4) consisted of 10 junctions of bismuth- 

 silver joined by silver solder and flattened out into rectangular 

 plates at the exposed junctions, which were blackened. The 

 sensitive area was 20 mm. long and 1*5 mm. wide, or a total 

 of 30 sq. mm. As the slit-width used in all the experiments 

 was only 1 mm., the effective area of the exposed junctions 

 was only 20 sq. mm. 



The galvanometer used was a modified form of the Thomson 

 galvanometer, and was specially designed by Paschen t for 

 radiometric measurements. The magnet system consisted of 

 two groups of thirteen magnets arranged alternately on 

 opposite sides of a fine glass stem and supported by a fine 

 quartz fibre. The coils wore elliptical in shape, and were 

 wound with six different sizes of wire with the object of 

 producing a. maximum field for a given resistance of copper. 

 The period could be controlled by means of a magnet, and 

 it was adjusted to have a full period of 5 sees. It was found 

 that while a longer period did not materially increase the 

 sensibility, it made the zero drift considerably greater. The 

 resistance of the thermopile was 2"93 ohms and that of the 

 galvanometer, with the coils connected in multiple series, 

 which was the arrangement always adopted, was 30 ohms. 

 The sensitiveness of the instrument was such that a deflexion 

 of 1 mm. on a scale at the distance of one metre was produced 

 by a current of '00025 microampere. 



One of the greatest difficulties met with in the work was 

 the variation produced by temperature changes and by stray 

 air currents. To overcome these the thermopile and slit 

 were enclosed in a nickelled box, shown at B in fig. 3, which 

 was both packed inside and surrounded outside with cotton- 

 waste. The whole spectrometer was enclosed in a wooden 

 box lined with absorbent cotton, and all the free space between 



* Paschen, Ann. d. Phys. xxvi. 1. pp. 120-138; 5. pp. 1029-1030 

 (1908). 



t Paschen, Ann. d. Phys. xxvii. 3. pp. 537-570 (1908). 



