DESCRIPTION OF APPARATUS AND METHODS. 



21 



This is the largest spectrometer }et set up for emission and absorp- 

 tion work. Greater sensitiveness of the radiometer would be necessary 

 in order to be able to use a spectrometer of greater dimensions. 



THE RADIOMETER. 



The construction of the radiometer is shown in fig. 3, which is a 

 section of the instrument at right angles to the one facing the spectro- 

 meter. It was built according to the design of Nichols.^ The outer 

 ^Topump ^^^^ ^^ ^ block of bronze, 5 by 5 by 10 cm., 

 with an axial boring 3 cm. in diameter. The 

 top was made from the neck of a large 

 round bottle, into which was fitted a glass 

 tube, which led to the exhaust pump. In 

 series with the radiometer and pump was 

 placed a tube containing gold foil on cotton 

 to absorb the mercury vapor, which was 

 found to electrify the vanes and thus cause 

 one of them to adhere to the window. As 

 a further precaution against electrification, 

 the inner rock-salt window was partly cov- 

 ered with tinfoil, which was in contact with 

 the outer case. (See Appendix III.) 



There were two lateral borings in the 

 outer case ; the one in front of the vanes was 

 2 cm. in diameter and admitted the energy 

 to be measured upon the exposed vane ; the 

 one in front of the mirror, not shown in the 

 figure, was long and narrow and was used 

 to read the deflections of the 



Th 



im 



,^- 



R, 



r 



(i77Z 



suspension. This window was 

 closed with plate glass. The 

 w i n d o w s were placed upon 

 ground surfaces of the casing, 

 which had been covered with a 

 mixture of beeswax and tallow. 

 Around the edges of the win- 

 dows was a layer of beeswax 

 covered with shellac. After the latter had dried there was no appre- 

 ciable leaking. 



Fig. 3. 



V=vanes-, M=miTTor; Ri,R2=rock-saIt windows 



'Nichols, E. F. : Phys. Rev., iv, p. 297, 1897. 



