172 RADIOMETRY. 



bolometer strip. It remains, therefore, to determine whether the present 

 bolometer behaves likewise, and also whether the same accuracy is attain- 

 able with the thermopile. 



To this end a bolometer was constructed with the greatest care. It 

 was annealed before adjusting the resistance of the strips, covered elec- 

 trolytically with platinum black (after the method of Kurlbaum) and then 

 smoked over wire gauze over a paraffin candle. The resistances of the 

 bolometer strips were 1.782 and 1.797 (A = 0.015) ohms, respectively. 

 After blackening them they were 1.766 and 1.818 (A = 0.052) ohms, re- 

 spectively. The width of the bright strips was 0.5 mm., which increased 

 to 0.56 mm. after blacking. The length was 11 mm. and thickness less 

 than 0.002 mm. The bolometer current was 0.04 ampere and throughout 

 the following experiments there was no difficulty due to air-currents, or 

 drift. Magnetic disturbances were at a minimum, and, as a whole, condi- 

 tions for accurate measurements were as perfect as one would expect. 



The thermopile of 0.08 mm. wire (20 junctions covered with a slit 

 0.5 mm. wide) already described, showed a slight lag in registering the 

 energy received. Although this was not marked, there was a tendency 

 for the deflection to creep, instead of stopping abruptly as in the case of 

 the bolometer. This was most marked in large deflections, and necessi- 

 tated exposing the thermopile to radiation for a definite time (6 seconds) 

 and taking the zero at the expiration of an equal interval of time. The 

 results are given in table VIII. The last two values for the thermopile are 

 vitiated by radiation from the rotating disk, to be noticed presently. The 

 results show that there is no great difference in the two instruments. It 

 was necessary, however, to note the time of exposure of the thermopile, 

 which is not convenient for large deflections. The estimation of the 

 relative merits of the bolometer and the thermopile is, therefore, a personal 

 one, and from the experience gained it may be said that for measuring in- 

 tense sources the bolometer is the more accurate (when working to 0.5 per 

 cent) unless great precautions be taken in making the thermopile readings. 

 The theoretical temperature sensitiveness of the thermopile was con- 

 siderably greater than that of the bolometer, as was found on subsequent 

 computation. It may be added, therefore, that if the bolometer sensitive- 

 ness had been increased, by increasing the current through it, there would 

 have been greater unsteadiness in the galvanometer readings. 



Experiment with a Sectored Disk. 



In comparing the relative merits of the bolometer and the thermopile, 

 the simplest method appeared to be to reduce the intensity of the source 

 by a known amount, by using a sectored disk, the angular openings of 

 which are accurately known. It will be noticed that while the ratios of 

 energy transmitted by the sectored disk were in close agreement in any 

 series of measurements (see tables VIII and IX) the numerical values were 



