  In carrying out calculations of the effect of longitudinal space charge on
longitudinal motion, the transverse beam size appears in a form factor which is
usually written as $g=1+2\ln (b/a)$. In fact, this expression applies to
particles with vanishing betatron amplitude in a beam with uniform transverse
distribution. It is argued that an average over the transverse distribution
should be used instead of the value on axis. It is shown that for the realistic
`binomial' family of distributions the 1 in the above expression for $g$ should
be replaced by a value near 0.5 if $a$ is interpreted as twice the rms width of
the beam.
