  The electron transmission through a {\it closed} Aharonov-Bohm mesoscopic
solid-state interferometer, with a quantum dot (QD) on one of the paths, is
calculated exactly for a simple model. Although the conductance is an even
function of the magnetic flux (due to Onsager's relations), in many cases one
can use the measured conductance to extract both the amplitude and the phase of
the "intrinsic" transmission amplitude $t_D=-i|t_D|e^{i\alpha_D}$ through the
"bare" QD. We also propose to compare this indirect measurement with the
(hitherto untested) direct relation $\sin^2(\alpha_D) \equiv
|t_D|^2/\max(|t_D|^2)$.
