154 RADIOMETRY. 



used a loop, one part of which was silver and the other was of palladium; 

 the latter used a junction of bismuth and antimony, which was soldered 

 to a loop of copper wire. 



The sensibility of the Boys instrument was given as To~ei>^~oTro to 

 9^~o'oV"^'0"o f x degree. From subsequent work with other radiation meters 

 in which this high degree of sensitiveness has never been attained, it would 

 appear that the sensibility of the radiomicrometer was overestimated. It 

 certainly has never attained the sensibility of the radiometer, one example 

 of which, used by Nichols (loc. cit., Table V), was twelve times as sensitive 

 as the radiomicrometer of Boys. The latter gave a deflection of a little 

 less than i cm. per square millimeter of exposed vane for a candle and 

 scale, each at a distance of i meter. Paschen 1 attempted to improve the 

 radiomicrometer, but out of about fifty junctions only three were useful, 

 and these were only three times as sensitive as that of Boys, while the 

 period was about 40 seconds. The long period is not always detrimental, 

 however, for the radiomicrometer is not subject to magnetic disturbances 

 and is a very useful instrument for work not requiring the highest attainable 

 sensitiveness. 



The writer has indicated further improvements in the instrument (see 

 Carnegie Publication No. 65) and places it in a vacuum, which increases 

 the sensibility by at least 70 per cent. The instrument was about six 

 times as sensitive as that of Boys for a full period of 25 seconds. Para- 

 and dia-magnetism limited the sensitiveness to this value. The work 

 with this instrument brought out the fact that one is inclined to use too 

 strong field-magnets, and that further progress can be made by using 

 weak magnets, or narrow strong magnet, situated as far as possible above 

 the thermo-junction, so as to avoid the effects of para- or dia-magnetism. 

 The combination of the radiomicrometer and the radiometer is feasible, 

 although the writer found its usefulness as limited as that of the radio- 

 micrometer. When wires can be obtained more free from magnetic 

 material it will possible to construct a more sensitive instrument. It is 

 doubtful, however, whether it will ever surpass the bolometer used with a 

 galvanometer of the highest sensibility. With the radiomicrometer, Lewis 2 

 was able to investigate infra-red emission spectra of the alkali metals, 

 which are weak in energy. Wilson 3 and Julius 4 have used the radio- 

 micrometer for total and spectral radiation work, and have found the 

 instrument highly satisfactory. The slow period and lack of portability, 

 mentioned by some writers, is certainly not to be weighed against its 

 indifference to magnetic perturbations and constancy of the zero reading. 

 Even a slow period is less objectionable than a quick period instrument 



1 Paschen, Ann. der Phys. (3), 48, p. 272, 1893. 



2 Lewis: Astrophys. J., 2, p. 1, 1895. 



3 Wilson: Proc. Roy. Soc, 55, 1894; 58, 1895; 60, p. 337, 1896. 



4 Julius: Handlingen 5, de Nederlandisch Natuur- en Geneeskundig Congres, 1895. 



