  We have built an atom interferometer and, by applying an electric field on
one of the two interfering beams, we have measured the static electric
polarizability of lithium with a 0.66 % uncertainty. Our experiment is similar
to an experiment done on sodium in 1995 by D. Pritchard and co-workers, with
several improvements: the electric field can be calculated analytically and our
phase measurements are very accurate. This experiment illustrates the extreme
sensitivity of atom interferometry: when the atom enters the electric field,
its velocity increases and the fractional change, equal to 4 x 10^(-9) for our
largest field, is measured with a 10^(-3) accuracy.
