PRESSURE GAUGES 23 



therefore such as would be produced at the base of a column of liquid of 

 specific gravity 1 S. This gives a difference of pressure of (1 S) h feet 

 of water, and the gauge reading is thus multiplied in the ratio 1 -=- 1 S, 

 as compared with that of a simple water gauge recording the same 

 difference of pressures. This multiplying factor will be termed the " gauge 

 coefficient" in future, and will be denoted by C. The three-way cock at 

 B allows of any air in the connecting pipes being readily expelled, and 

 allows the quantity of air or of oil in the upper part of the tubes to be 

 regulated as required. 



A second type of gauge, in which the gauge fluid is heavier than water, 

 is shown in Fig. 10 A or B. If the pressure difference is large, mercury 

 may be used in the gauge, in which case if h is the difference of level, the 

 difference of pressure equals (13*596 1) h = 12'596 h feet of water, and 

 the gauge coefficient = *0794. If the pressure difference is small, a 

 mixture of carbon tetrachloride and of gasoline with a specific gravity of 

 1*25 gives a gauge whose coefficient is 1 -f- (1*25 1) = 4. 



A type of differential gauge which is useful for measurements of air 

 flow is shown in Fig. 10 C, and consists of a U-tube whose branches are 

 enlarged at their upper ends. Water is poured into one and some lighter 

 fluid (specific gravity S) into the other leg of the tube. Let a and A be the 

 sectional area of the small and large portions of the tubes. If is the 

 zero of the scale reading, -let 0, B and C be the levels of the surface of 

 separation of oil and water and of the free surfaces when the latter are 

 exposed to atmospheric pressure, and let 0', B', C' be these levels when 

 the pressure difference at the free surfaces is B p, these pressures being 

 respectively p and p + B p. 



Then p + h' w = p + B p + Sh' 



But h'. = h + x- x a . = h + x ( 1 - a 4 ). 



<i \ A / 



And h' w = h w + x + x? r =h w + x(l + -\ 



si \ A / 



As compared with a simple water gauge, the coefficient in such a gauge 



By using two fluids of specific gravity S' and S, such as a mixture of 



