114 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



indicated by the little circles. The evidence is of the same nature, remember- 

 ing that, quantitatively, the effect must depend on the rapidity of exhaustion, 

 etc., which is kept low to avoid influencing the needle by a current of air 

 directly and also to avoid large temperature decrements. 



There are thus two radiation effects, respectively positive and negative, 

 which pass continuously into each other. One of these may be ascribed to the 

 change of pressure into kinetic energy on the hotter side or side of convection 

 excess; i. e., there is pressure deficiency on the hotter side. The other, at higher 

 vacua, is the radiometer effect, which is of opposite sign; i. e., there is pressure 

 excess on the hotter side. Hence, between the two, somewhere near 65 cm. 

 here, the two phenomena are in equilibrium and the effect of exhaustion is nil. 



The investigation resulting in figure 144 was merely made for practical 

 purposes and the deflections depend on many subsidiary details. It will be 

 carried out under standard conditions presently. 



There is a correlative test for the effect of radiation, from without, on the 

 needle in an exhausted case. To apply this, the case was exhausted to above 

 70 cm. and a ball heated to 60 or 70 placed outside. No discernible effect 

 was produced, or, in other words, the effect of external radiation is now small. 

 Consequently the drift of the needle must vanish and the triplets become static 

 repetitions of each other. That this is the case the experiments of the next 

 paragraph will show. It follows, also, that the resistance encountered by the 

 needle is now referable to the viscosity of air and therefore possibly open to 

 computation. 



At a later opportunity I returned to these interesting phenomena again, with 

 the object of obtaining more definite results. The case of the needle was, 

 as above, exhausted in steps of 10 cm. of mercury, successively and quite 

 slowly, so as not to disturb the motion of the needle by air-currents. For this 

 reason, pressure increments of 10 cm., *. e., the return passage, did not suc- 

 ceed very well, as the needle was apt to be influenced by any accidental puff 

 of air. During exhaustion there is no such danger. 



Experiments were first made with the ball M removed, and they are given 

 in figure 145 b. After each exhaustion the excursion of the needle was allowed 

 to return to the equilibrium position, marked with little circles on the diagram. 

 The period observed in these cases was relatively short and did not exceed 

 six minutes, three minutes for the semi-period. Moreover, the equilibrium 

 position varies somewhat, possibly because more than one swing should have 

 been waited for; but this would have enormously extended the experiments 

 and introduced other drift-like errors. 



