156 RADIOMETRY. 



The problem in thermopile construction is to have it of low resistance 

 (equal to that of the galvanometer), of low heat capacity and heat conduc- 

 tivity, and of high thermoelectric power. The latter requirement is fulfilled 

 by using iron and constantan. The heat capacity can be reduced by 

 using finer wire, say 0.06 to 0.1 mm. diameter, and by making the unex- 

 posed junctions smaller than the ones to be exposed. The latter are 

 soldered with quite large beads of silver, which are then flattened to present 

 a large surface. The unexposed junctions do not need this, and the small 

 bead formed by the fusion of the two wires (with a bit of silver solder if 

 necessary) can be hammered thin, in order to have it radiate rapidly. By 

 using finer wires the resistance will be increased if the dimensions of the 

 Rubens pile be retained. In the commercial instrument, at least one- 

 third of the wire is between the unexposed junctions and the binding-posts. 

 The greater part of this wire may be eliminated by making the supporting 

 frame narrower, while still retaining the original distance between the 

 exposed and the unexposed junctions. The elimination of this superfluous 

 wire will reduce the resistance by about one-third. 



Comparison of Old and New Form of Thermopile. 



In order to test these conclusions in regard to the use of finer wire, a 

 new iron-const ant an pile of 20 junctions, made of wire 0.08 mm. diameter, 

 was ordered from the makers of the original instrument. Although the 

 specifications were not completely fulfilled (the frame was nearly the same 

 size as the original, which increased the resistance to 9 ohms), the sensi- 

 tiveness was 1.4 times that of the old type which has resistance of 4.8 ohms 

 (wire about 0.15 mm.). By means of suitable switches the two thermo- 

 piles were connected to the same galvanometer, having a full period of 

 12 seconds (i=2Xio~ 10 amp.) and exposed to the radiation from a Nernst 

 heater. For all deflections, as large as 35 cm., the new thermopile showed 

 no drift greater than 2 mm., which may be attributed to the galvanometer. 

 On the other hand, the zero of the old thermopile would drift 0.5 cm. in 

 a 10 cm. deflection to 2.2 cm. in a 27 cm. deflection and would require 

 16 to 20 seconds for the deflection to return to its original zero. 



The two instruments were then tested in a vacuum. The sensitiveness 

 of the old instrument was increased only 15 per cent, while no change in 

 sensitiveness could be detected in the new one, although two distinct tests 

 were made on different days, the pressure having been reduced to 0.01 mm. 



The thermopiles are mounted on ivory frames and are covered with a 

 sheet of copper, one side having a slit, the other a funnel-shaped opening 

 (1 by 15 mm.) in it. The slit was covered and the radiation passed through 

 the funnel. The whole was suspended from a rubber cork in a wide- 

 mouthed bottle, which was exhausted with a mercury or a Geryk pump. 

 The source of energy was an incandescent lamp. With 200 ohms in 

 series with the galvanometer the deflections were about 10 cm. The fact 



