l6o RADIOMETRY. 



In his earlier communications the writer held to the belief that the 

 weight of the vanes and their size were the most important factors in 

 determining the sensitiveness and period of a radiometer. However, so 

 many factors enter into the problem that it is difficult to decide this point, 

 and the following test may be of interest, showing that the sensitiveness is 

 determined principally by the diameter of the quartz fiber suspension. 



Comparison of Sensitiveness and Diameter of Fiber Suspension. 



Using the same vanes (0.5X9 mm. area) the sensitiveness and period 

 were found for a heavy and a light quartz fiber suspension. The main 

 difficulty was to insure that the vanes were at the same distance from the 

 window, and that the pressure was the same in the two cases. Hence these 

 qualities are only approximate. In table V, it will be noticed that in 

 changing from a heavy to a light fiber the sensitiveness is increased 4.5 

 times while the period was increased almost threefold. It further shows 

 that for the same (light) fiber the sensitiveness was doubled (36 to 71 cm. 

 per mm.) by changing the pressure and the distance from the window, 

 which was of fluorite, and hence opaque beyond 10 /i. The sensitiveness 

 of 71 cm. per square millimeter of exposed area is the highest on record. 

 This, however, was not the maximum sensitiveness since the pressure was 

 0.02 mm., while radiometers have their maximum sensitiveness at a pres- 

 sure of about 0.05 to 0.1 mm. At this pressure, however, heat conduction 

 would cause annoyance. 



The vanes of this suspension were of platinum foil 0.01 mm. thick, 

 covered on one side electrolytically with platinum black, and then smoked 

 over a candle. (It is best to cool the gases from the flame by placing a 

 wire gauze or sheet of metal full of holes between the flame and the vanes 

 when smoking them.) These vanes were suspended by means of one of 

 the finest workable quartz fibers, and when within 3 mm. of the window 

 either one of the vanes would always approach and adhere to it, even at 

 atmospheric pressure. From tests with fluorite windows, which from 

 internal strains might be piezoelectric, and with rock-salt windows, when 

 bare, and also when covered with tinfoil, it was found that this effect is 

 not due to electrification. Starting with the vanes parallel, and at a dis- 

 tance of about 5 mm. from the window, it was found that as this distance 

 was decreased one of the vanes (generally the one to be exposed to radia- 

 tion) would approach the window, and for a distance of about 3 mm. 

 would turn until the plane of the vanes was at right angles to the window. 

 The observations extended over several months, and all evidence indicates 

 that this effect is due to gravitational attraction. As a result of this the 

 deflection of such a vane would not be proportional to the energy received. 

 The radiometer had no torsion-head to control the zero. However, for 

 general work with very sensitive radiometers a torsion-head would be nec- 

 essary, since the best pumps may leak, which will cause a slow "drift." 



