Nipher — On Pressure Measurements tn a Fluid Stream. 27 



Pitot tube. This would indicate that for ordinary velocities, 

 if the caps on the ends of the tubes were replaced by short 

 L tubes with open ends turned in the direction of rotation, 

 approximate equilibrium should result. 



This was tested by means of a tube 2x92.5 cm. long, bal- 

 anced on its axis of rotation. It was clamped to the end of 

 a small pulley of an electric motor. The connection through 

 the water joint with the axis of a water column having a rise 

 of 1 in 20, gave a very sensitive indication of the pressure at 

 the axis. With a rotation of 7i=b per second a decrease' of 

 12 to 15 cm. in the scale reading was found. When a cur- 

 rent of air was allowed to drift across the plane of rotation 

 the decrease of pressure diminished to one or two cm. The 

 residual decrease in pressure was in part due to this current 

 of air drifting across the mouths of the tube, and in part to 

 the vortex of the air set up by the motion of the tube itself 

 which was not wholly eliminated by the drift of air. 



It is evident that such a method could not be used for the 

 purpose of determining the value of the constant for the 

 Pitot tube. It can only serve as a rough check on other 

 methods. 

 *If Newton's equation be assumed to represent the pressure 

 into the open mouth of the revolving tube, when it is moving 

 with a velocity of v, the pressure shown by a gauge connected 

 at the axis of rotation should be by (1). 



P* v^ ~2CT , 2CT 



b 2CT 



e 



+ 6 "^"—1. (6) 



The terms of this equation may be represented by the fol- 



lowing series : — 



2CT 



JOT'^ 2AC^T^ "~ 2A.6C^T^ '^ " ' 



+ 



2CT'' ~ 2CT~ {2CTf^ {2CT) 



