  In this paper it is shown that a measurement of the relative luminosity
changes at the LHC may be obtained by analysing the currents drawn from the
high voltage power supplies of the electromagnetic section of the forward
calorimeter of the ATLAS detector. The method was verified with a reproduction
of a small section of the ATLAS forward calorimeter using proton beams of known
beam energies and variable intensities at the U-70 accelerator at IHEP in
Protvino, Russia. The experimental setup and the data taking during a test beam
run in April 2008 are described in detail. A comparison of the measured high
voltage currents with reference measurements from beam intensity monitors shows
a linear dependence on the beam intensity. The non-linearities are measured to
be less than 0.5 % combining statistical and systematic uncertainties.
