174 



RADIOMETRY. 



values from 69.3 to 77.2 per cent. In fig. 105 are plotted the galva- 

 nometer deflections for different distances of the disk (abscissae) from the 

 screen. The latter was 80 cm. (source at 2 m.) from the bolometer, and 

 had an opening in it which was the size of the openings in the sectored disk. 

 No radiation was observed from the stationary disk, nor from the shields 

 back of it when the open sector was before the bolometer. The speed of 

 the disk was such as is used in photometry, and was kept constant for 

 each series of observations. In the lower curve for the 240 disk the 

 speed was slow and there was a flicker on viewing it. The curves show 

 that the maximum radiation occurs when the disk is about 6 cm. from 

 the shield; and it disappears immediately on stopping the disk. 



Table IX. Reliability of Bolometer Measurements. 



a Mean of 6 to 10 readings. 



6 Galvanometer period 8 seconds, vibration undamped; 20 ohms in series with galvanometer. Temperature 

 sensitiveness =6X io 6 C High values due to radiation from moving disk, which is 0.5 m. from screen. 



c Galvanometer period 14 seconds and vibration just damped. Temperature sensitiveness =i.2Xio- S C 



d Nernst heater on 98 volts at 1 m. from bolometer. Galvanometer period 8 seconds. 30 ohms in series with 

 galvanometer. Total deflection about 80 cm. Individual deflections vary 0.2 to 0.5 per cent from mean. 



e Galvanometer period 14 seconds. 



/ Galvanometer period 8 seconds. Disk better shielded than in previous experiments and closer to screen 

 6 cm. from it. Heater 150 cm. from bolometer; screen at 80 cm. 



g 14 ohms in galvanometer circuit. Galvanometer period 14 seconds. Nernst heater on 74 volts. 



h No resistance in galvanometer circuit; results show that high value is not due to lack of proportionality of 

 galvanometer deflections. 



* Nernst heater on 95 volts. 20 ohms in galvanometer circuit. Total deflection about 45 cm. 



The motor was run continuously for a complete series of measurements 

 and no deflections greater than i to 2 mm. were observed from it or the 

 disk, immediately after stopping the rotation. In these tests the motor 

 was shielded from the bolometer. On removing the shield and the disk 

 and on running the motor the deflections were from 1 to 3 mm. The 

 whole shows that the sectored disk is not as applicable as one would sup- 

 pose, unless one determines the corrections which in two series of experi- 

 ments were found to be in very close agreement. 1 In the present curves 



1 The sector was rotated at a higher speed than actually required with a bolometer. By 

 means of suitable pulleys the speed may be reduced to perhaps 35 that used in the present 

 test, which would decrease the errors. 



