221 



where T^' and 7'," represent the temperatures of outer wall and 

 mica plate (properly speaking tube B) resi)ectively in the experiment 

 at llie ordinary temperature, 7'./ and 7V the corresponding tempe- 

 ratures in the experiment at higher temperature, c the value of the 

 reduction factor calculated from the dimensions of the apparatus 

 (^as we have seen this calculation can only give an approximate 

 value), «, and a^ tlie two elongations. The second column gives 

 the ditferences of the successive p:-essures, the third the same ditFe- 

 rences according to the observations w^ith the pipettes. 



In the first series the limits of proportionality are evidently far 

 exceeded: it can therefore serve as a calibration of the manometer 

 as a pressure-indicator. The second series shows clearly that at first 

 the absolute manometer gives greater values for the pressures than 

 the pipettes, which are however in a constant ratio to the latter, 

 while later on its readings are lower than those of the pipettes, as 

 might be expected^). It is also intelligible that our manometer gives 

 higher readings in the beginning than the pipettes considering that 

 the distance between the windows and the mica plate cannot be 

 taken as small as compared with the width of the windows, so that 

 parts of the movable mica plate outside the perpendicular projection 

 of the windows will also be struck by molecules which proceed 

 from the heated wall. This action along the edges owing to which 

 c can only be approximately calculated was considerable in our 

 case; its influence on the pressure follows immediately from the above 



1 



calibration numbers: obviously c has to be replaced by c = 



0.881 



1.135 c. 



^ 4. IVte measure nients. 



a. The manometer mounted as in lig. 2 was then exhausted for 

 a long time under strorig heating to 300° C. : for this purpose it 

 was joined to a GaedI', molecular airpump, for the loan of which 

 we are indebted to the kindness of Lkybolds Nachf. of Cologne, which 

 we here gratefully acknowledge. As aj)pears from the measurements 

 communicated below it is j>ossible to use this excellent, reliable and 

 easily worked instrument continuously at pressures of 0,007 barye 

 without the least trouble. After having been connected to the pump 

 at the tem[)erature mentioned for about a day the manometer ceased 

 to give oft' gas. The vacuum had become constant. The small 

 remaining pressure was thereupon measured, while the vapour 



') Debi.te (Physik. Zs. 1911, p. 1115) has tried to I'epresent lliis behaviour by 

 a formula of approximation. 



