THE BOLOMETER. 



173 



in all cases higher than the true ones. 



In other words, the disk transmitted 

 too much energy, or the apparent opening was larger than the true one. 

 It remained, therefore, to be shown whether this was due to diffraction (of 

 the very long wave-lengths) or to lack of proportionality in the registering 

 of the energy by the bolometer and the thermopile. The method of ob- 

 servation consisted in taking from 5 to 10 readings without the disk, then 

 a similar number with the rotating disk interposed, followed by a number 

 without the disk. 



* Nernst heater on 98 volts at 1 m. from bolometer. Galvanometer (full) period 8 seconds undamped; 50 ohms 

 in series. Total deflection about 80 cm. Bolometer is perfectly steady and comes to rest abruptly. Readings 

 vary from 0.1 to 0.7 per cent from mean of about 10 in each set. Temperature sensitiveness =6X 10 5 . 



t Conditions same as for bolometer. Thermopile deflection "creeps," and does not come to rest in same time 

 as galvanometer (on open circuit or with bolometer), due to its larger heat capacity. Galvanometer single swing 

 of 4 seconds increased to 6 seconds and is fully damped, due to lag of thermopile; 50 ohmsin series with galvanom- 

 eter. Temperature sensitiveness = 2X io 6 . 



J Source and disk nearer screen. Difficult to read deflection on account of "creeping," which amounts to 1 to 

 3 mm. Readings made at end of 6 seconds vary by 0.4 per cent from mean; 20 ohms in series with the galvanom- 

 eter. Temperature sensitiveness = 5X io 6 . 



The first test made was to determine whether the rotating disk (30 cm. 

 diameter, 1.3 m. from the bolometer) affected the instrument; and it was 

 found that the resulting deflections 1 to 2 mm. were no larger than those 

 due to stray radiation reflected from the stationary disk. A heavy black 

 cardboard shield was then placed between the bolometer and the disk 

 (0.5 m. from the disk) and similar screens were placed around the source, 

 which was 2 m. from the bolometer. No radiation was detected from the 

 stationary disk, whether the open or closed part of the disk faced the 

 bolometer; but unfortunately this test was not made for the moving disk. 

 The disk with the 240 opening gave values 0.5 per cent too high (see 

 table IX). The results with the 120 disk (6 openings of 20 each) were 

 in still greater error. The space between the bolometer and the shield 

 was then entirely inclosed, and with the disk close to the opening (7X10 

 cm.) in the shield the discrepancy became still greater. It was then found 

 that the increased transmission is due to the moving disk and depended 

 upon the distance of the disk from the screen. 



It was further shown that the transmission was proportional to the 

 speed, so that the 240 disk (true transmission 66.827 per cent 1 ) gave 



1 These disks and their constants were supplied by Dr. Hyde. Bureau of Standards, 

 Bulletin, 2, p. i, 1906. 



