164 RADIOMETRY. 



ness which was in arbitrary units. The same may be done with the radi- 

 ometer. Its sensitiveness is easier to control, since it can be made to 

 depend only upon the pressure of the residual gas the constant of a 

 galvanometer varies continually. It is not affected by magnetic effects, 

 and a heavy vane is less affected by earth tremors than is a very light 

 galvanometer suspension. It is sensitive to temperature changes, but less 

 so than the bolometer, and it can be more easily shielded from temperature 

 changes than can a bolometer with its galvanometer, battery, etc. The 

 fact that it is not portable is not a serious drawback, since it is not usually 

 necessary to move the instrument. It has two disadvantages, viz, its 

 window, or preferably double window, is selective in its transmission, and 

 its period is somewhat longer than that of a bolometer and galvanometer 

 of equal sensitiveness. But the latter is nearly always drifting and to 

 repeat one's readings it takes as long for an observation as it does with a 

 radiometer. 



Since the weight is of minor importance, tremors are avoided by having 

 the suspension weigh about 8 to 10 mgs. When used with a good mercury- 

 pump, it requires no attention after it is properly adjusted. A delicate 

 galvanometer requires frequent adjustment and in connection with a 

 bolometer the investigator's time is occupied principally with the care 

 of the instrument (at least that has been the writer's experience), which 

 should be a secondary matter. The two instruments are of the same 

 order of sensitiveness, with the possibility of the radiometer being the 

 more sensitive. 



This is well illustrated in the test for their efficiency to ultra-violet 

 radiation, where both instruments were at about their maximum working 

 sensitiveness. The bolometer used was 0.22 by 10 mm. in area, resistance 

 2.8 ohms, and for a battery current of 0.04 amp. with a galvanometer 

 sensitiveness of i=i.5Xio~ 10 amp. (period 16 seconds) had a tempera- 

 ture sensitiveness of 9X10"" per millimeter deflection, on a scale at 1 m. 

 (see table VII). (A Nernst heater was also used in making the com- 

 parison.) 



A candle at 1 m. gave a deflection of 45 cm. which, on the assump- 

 tion that the sensitiveness is proportional to the square root of the area 

 of bolometer strip, is 30 cm. per square millimeter. For the radiometer 

 having a vane 0.5 by 9 mm. a candle gave a deflection equivalent to 159 cm. 

 at 1 m. or, since the deflection is proportional to the area of the exposed 

 vane, 35 cm. per square millimeter (table VI). In other words, the radi- 

 ometer was 1.2 times as sensitive as the bolometer, or 1 mm. deflection 

 corresponded to 7.5Xio~ 6 C. (For a full period of 2.5 minutes its sensi- 

 tiveness was 3. 8 Xio~ 8 C.) Its period, however, was 4.5 times that of 

 the bolometer. This estimation of sensitiveness is based on the assump- 

 tion that the radiometer was as complete an absorber of energy as the 

 bolometer. Judging from its period, its efficiency is much lower than 



