the Value of the Pltot Constant. 



363 



2. When a pi tot is placed in a non-uniform flow the pitot 

 pressure which is measured is an average taken over the 



Curve V. 











/ 



1-2. - 







/ 





\-0 - 







V 





' 











0-8- 



















o-6- 













4 

















C-lr 



o/ 









/ 











<L. . _. ... 









10 20 30 C>*Sec' 



"hxea-n. Velocity of 3-£o\s 



area o£ the end of the pitot, and differs from that at its centre 

 by an amount depending upon the area of the pitot and the 

 distribution of velocity over its end. The amount of this 

 error was calculated assuming that the velocity changed 

 linearly across a diameter of the pitot, but it was found to be 

 quite negligible at low rates of flow for the size of pitot 

 used. 



3. It seemed feasible that superposed on the main 

 disturbance of the flow in the neighbourhood of the pitot 

 there might be a smaller secondary disturbance which, if it 

 depended on a power of the velocity less than the square, 

 might become comparable with the observed pressure when 

 the velocity was very low, even though it were negligible at 

 higher velocities as seemed likely from the work of other 

 experimenters. 



Definite evidence, however, has been obtained against this 

 view for the case of the round pitot used in the above. Thus 

 the pressure readings were unaltered by threading over the 

 pitot a piece of rubber tubing so that the whole pitot stem 

 was thickened about 4 diameters. 



2B2 



