APPENDIX IV. 

 A VACUUM RADIOMICROMETER. 



Radiometers having vanes weighing less than 10 mg. are easily 

 affected by earth tremors, so that for general work it is not advisable 

 to reduce the weight much below this amount. Of course, such a mov- 

 ing system is not very sensitive for a short period. For such a vane, 

 with a fiber suspension of such a diameter that the maximum deflection 

 is reached in 15 to 20 seconds, the sensitiveness is not much greater 

 than 9 to 12 cm. deflection per square millimeter surface exposed the 

 candle and scale each being at a meter's distance. However, a radio- 

 meter of such a sensitiveness always gives reliable readings and is to 

 be preferred to a more sensitive instrument which gives larger deflec- 

 tions, but which on account of the lightness of the vanes is affected by 

 earth tremors, temperature changes, etc. 



On the other hand, in the radiomicrometer of Boys, the period is 

 governed by the magnetic moment of the moving parts, rather than by 

 the torsion of the fiber suspension, and in nearly all instruments, thus 

 far described, is much shorter than that of the type of radiometer just 

 mentioned. This instrument has received but scant attention since it 

 was first described by Boys, who had a sensitiveness of only about 

 i cm. per square millimeter (candle and scale at I in.) for a suspension 

 weighing 32 mg. and a period of 10 seconds. He used a window to 

 prevent air currents. Paschen 1 attempted to improve the instrument, 

 but for the best junctions (out of some 50) he could obtain only about 

 three times the sensitiveness of Boys, for a period of about 40 seconds. 

 He then turned his attention to the bolometer, with his well-known 

 success. 



The bolometer, however, with a delicate galvanometer requires an 

 elaborate installation, and needs so much attention that the investiga- 

 tor's time is occupied principally with the care of the instrument, which 

 should be a secondary matter in any work. 



A great many radiometric investigations do not require the highest 

 attainable sensitiveness (e. g., the author's present work), and in such 

 cases a convenient accessory to a laboratory is a radiometer or a radio- 

 micrometer. The thermopile, of course, also has its place, but here heat 

 conduction, local e. m f.'s, etc., must be contended with. 



'Paschen: Ann. der Phys. (3), 48, p. 275, 1893. 



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