  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 features include a 4 T superconducting solenoid with 6-m-diameter
by 12.5-m-length free bore, enclosed inside a 10,000-ton return yoke made of
construction steel. The return yoke consists of five dodecagonal three-layered
barrel wheels and four end-cap disks at each end comprised of steel blocks up
to 620 mm thick, which serve as the absorber plates of the muon detection
system. To measure the field in and around the steel, a system of 22 flux loops
and 82 3-D Hall sensors is installed on the return yoke blocks. A TOSCA 3-D
model of the CMS magnet is developed to describe the magnetic field everywhere
outside the tracking volume measured with the field-mapping machine. The
magnetic field description is compared with the measurements and discussed.

Instrumentation and Detectors