  The Compact Muon Solenoid (CMS) is a general-purpose detector designed to run
at the highest luminosity at the CERN Large Hadron Collider (LHC). Its
distinctive featuresinclude a 4 T superconducting solenoid with 6 m diameter by
12.5 m long free bore, enclosed inside a 10000-ton return yoke made of
construction steel. Accurate characterization of the magnetic field everywhere
in the CMS detector is required. During two major tests of the CMS magnet the
magnetic flux density was measured inside the coil in a cylinder of 3.448 m
diameter and 7 m length with a specially designed field-mapping pneumatic
machine as well as in 140 discrete regions of the CMS yoke with NMR probes, 3-D
Hall sensors and flux-loops. A TOSCA 3-D model of the CMS magnet has been
developed to describe the magnetic field everywhere outside the tracking volume
measured with the field-mapping machine. A volume based representation of the
magnetic field is used to provide the CMS simulation and reconstruction
software with the magnetic field values. The value of the field at a given
point of a volume is obtained by interpolation from a regular grid of values
resulting from a TOSCA calculation or, when available, from a parameterization.
The results of the measurements and calculations are presented, compared and
discussed.
