The filament stretching device which is used increasingly as an appara
tus for measuring extensional properties of polymeric liquids is analy
sed. A force balance that includes the effects of inertia and surface
tension is derived. The force balance may be used to correct for the e
ffects of inertia and surface tension, provided online measurements of
the filament surface shape are available. In addition, the question o
f initial asymmetry due to gravity is addressed.