The difference between the velocity of discharge and the velocity due to the head may be reckoned in another way. The total height h causing outflow consists of two parts—one part he expended effectively in producing the velocity of outflow, another hr in overcoming the resistances due to viscosity and friction. Let
hr = crhe,
where cr is a coefficient determined by experiment, and called the coefficient of resistance of the orifice. It is tolerably constant for different heads with well-formed orifices. Then
va = √2ghe = √ { 2gh / (1 + cr) }.
(4)
The relation between cv and cr for any orifice is easily found:—
va = cv√2gh = √ { 2gh / (1 + cr) }
cv = √ { 1 / (1 + cr) }
(5)