  A new method of accurate calculation of the coefficient of viscosity of a
test liquid from experimentally measured terminal velocity of a ball falling in
the test liquid contained in a narrow tube is described. The calculation
requires the value of a multiplicative correction factor to the apparent
coefficient of viscosity calculated by substitution of terminal velocity of the
falling ball in Stokes formula. This correction factor, the so-called viscosity
ratio, a measure of deviation from Stokes limit, arises from non-vanishing
values of the Reynolds number and the ball/tube radius ratio. The method, valid
over a very wide range of Reynolds number, is based on the recognition of a
relationship between two measures of wall effect, the more widely investigated
velocity ratio, defined as the ratio of terminal velocity in a confined medium
to that in a boundless medium and viscosity ratio. The calculation uses two
recently published correlation formulae based on extensive experimental results
on terminal velocity of a falling ball. The first formula relates velocity
ratio to Reynolds number and ball-tube radius ratio. The second formula gives
an expression of the ratio of the drag force actually sensed by the ball
falling in an infinite medium to that in the Stokes limit as a function of
Reynolds number alone. It is shown that appropriate use of this correction
factor extends the utility of the technique of falling ball viscometry beyond
the very low Reynolds number 'creepy flow' regime, to which its application is
presently restricted. Issues related to accuracy are examined by use of our own
measurements of the terminal velocity of a falling ball in a narrow tube and
that of published literature reports, on liquids of known viscosity
coefficient.
