^{68}To give some perspective on just how little the liquid level changes in the well, consider a well-type manometer with a
1/4 inch (inside) diameter viewing tube and a 4-inch diameter circular well. The ratio of diameters for these
two liquid columns is 16:1, which means their ratio of areas is 256:1. Thus, for every inch of liquid motion in
the viewing tube, the liquid inside the well moves only 1 _
256 of an inch. Unless the viewing tube is quite tall,
the amount of error incurred by interpreting the tube’s liquid height directly as pressure will be minimal –
quite likely less than what the human eye is able to discern on a ruler scale anyway. If the utmost accuracy
is desired in a well manometer, however, we may compensate for the trifling motion of liquid in the well by
building a custom ruler for the vertical tube – one with a 255
256 reduced scale (so that 255
256 of an inch of liquid
motion in the tube reads as exactly 1 inch of liquid column) in the case of the 1/4 inch tube and 4 inch well
dimensions.