184 



THE ROYAL SOCIETY OF CANADA 



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 



r-io^^ 



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^ for radiometry 

 measurements. The magnet system consisted of two groups of thir- 

 teen magnets arranged alternately on opposite sides of a fine glass 

 stem and supported by a fine quartz fibre. The coils were 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 3-0 

 ohms. The sensitiveness of the instrument was such that a de- 

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

 by a current of -00025 micro ampères. 



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 

 iPaschen, Ann. der. Phys. 27, 3. pp., 537-570, 1908. 



