The unit of viscosity is a poise, 1 gm cm-1
s-1 = 10-1 kg m-1 s-1.
To measure viscosity, one must determine
flow in the presence of a velocity gradient.
Poiseuille2 derived a formula for the volume, ΔV,
of an incompressible fluid of viscosity, η, moved
through a tube of length l and radius, r, in a time, Δt, subject to a pressure drop, ΔP,