  The Pierre Auger Observatory is designed to study cosmic rays of the highest
energies ($>10^{19}$ eV). The ground array of the Observatory will consist of
1600 water Cherenkov detectors deployed over 3000 km^2. The remoteness and
large number of detectors require a robust, automatic self-calibration
procedure. It relies on the measurement of the average charge collected by a
photomultiplier tube from the Cherenkov light produced by a vertical and
central through-going muon determined to 5 - 10% at the detector via a novel
rate-based technique and to 3% precision through analysis of histograms of the
charge distribution. The parameters needed for the calibration are measured
every minute, allowing for an accurate determination of the signals recorded
from extensive air showers produced by primary cosmic rays. The method also
enables stable and uniform triggering conditions to be achieved.
