THE RADIOMETER. 1 63 



passed through a quartz cell 8 mm. thick, containing distilled water which 

 absorbed all of the infra-red energy. The observations consisted in ob- 

 taining the ratio of energy transmitted by a glass plate 8 mm. thick (which 

 is opaque to rays shorter than 0.3 p) to the total energy of the spark. 



Unfortunately, at the high sensitiveness required for ultra-violet radia- 

 tion the two instruments were not in perfect working order at the same 

 time. During the first test the bolometer had a short period, 10 seconds, 

 and caused trouble by the "drifting" of the zero with changes in the 

 spark, while in the second test the radiometer was leaking slightly, which 

 caused its zero to "drift." Then, too, the spark was by no means constant, 

 but, as will be seen presently, the ratio above referred to is about the same 

 for the radiometer and the bolometer, after correcting for the loss of 4 

 per cent by reflection at the nuorite window. The agreement is close 

 enough to show that the radiometer is not selective in its action and hence 

 is adapted to investigations in the ultra-violet. 



In the first test the direct radiation from the aluminum spark (with 

 condenser) was compared with the part transmitted by glass. There was 

 some infra-red energy in this case which made the ratio lower than in the 

 second experiment. The direct deflections with the radiometer were 

 about 20 cm. The ratio of the deflection through a piece of plate glass 

 to the direct deflection varied from 17 to 19.5 per cent (mean about 18 

 per cent), while with the bolometer the same ratio varied from 16 to 20 per 

 cent, the mean being about 19 per cent. The bolometer followed the 

 flunctuations of the spark, hence the greater variations. The spark was 

 75 cm. from the radiometer and 25 cm. from the bolometer. 



In the second test the infra-red radiation was absorbed by a cell of 

 water, with quartz windows. Plate glass was again used to absorb the 

 ultra-violet. In this test, the ratio of the radiation transmitted by the 

 glass to the total radiation was on an average about 65 per cent, while 

 the same ratio for the bolometer was 67 per cent. Correcting for the 

 loss by reflection at the window the ratio for the radiometer would be 

 about 69 to 70 per cent. 



The results show that the radiometer is not selective, i. e., it is as efficient 

 in the ultra-violet as in the bolometer. 



THE RADIOMETER COMPARED WITH THE BOLOMETER. 



In discussing the merits of the radiometer, writers have generally 

 emphasized the fact that the radiometer is not adapted for quantitative 

 work since it cannot be calibrated. As a matter of fact, in reviewing the 

 work done in radiation it was found that even with the bolometer there 

 are only a few cases where the energy was obtained in absolute measure. 

 Even in the study of the laws of radiation from a hollow enclosure or 

 KirchhofT radiator (so-called "black-body"), the direct galvanometer 

 deflections were observed and reduced to a single standard of sensitive- 



