498 



Mr. J. D. Fry on a New Micromanometer. 



and for this purpose it is necessary to apply to it a series of 

 very small known differences of pressure. The author has 

 found the following method convenient for this purpose : — 



The two limbs of an inverted U-tube (fig. 2) containing 

 air are kept at a fixed difference of temperature. This causes 



Fig. 2. 



(^ 



a small difference of pressure between their lower ends if the 

 latter are connected to opposite sides of the manometer. 

 This U-tube with its jackets is mounted on a board hinged 

 so that the plane of the U niay be inclined at any angle to 

 the vertical, thus making it possible to obtain a series of 

 pressures ranging from zero when the limbs of the U are 

 horizontal to a maximum when they are vertical. 



The complete expression for the difference of pressure 

 applied is then : — 



p=KUr — FjTJcos0; 



where 



*■"" 760 ' 



L = Length of each limb of the U-tube, 

 Ao= Density of air at N.P.T., 

 B = Height of barometer, 

 T x and T 2 = Thc absolute temperatures of the two limbs, 

 = The angle at which the U-tube is inclined 

 to the vertical. 



