The longitudinal coupling impedance in modern storage
rings has to be limited to ensure beam stability. To de-
termine the real part of the narrow band impedance we
suggest to measure the loss factor of the resonant modes
using short electron bunch passing through the tested el-
ement. This method is also suitable for the measurement
of the longitudinal impedances for nonrelativistic particles,
that is important for some modern projects of ion storage
rings designed to achieve ordered structure in the ion beam.
The desired value of relativistic parameter gamma can be chosen
by adjusting energy of the electron bunch. The impedance
of the fundamental mode of the wall current monitor was
measured using 60 ps electron bunch from photogun. The
dependence of impedance on beam energy is measured in
the range 25-50 kV . Comparison of experimental data
with computer simulation is done