  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 flux 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 that was measured with the field-mapping
machine. The voltages induced in the flux loops by the magnetic flux changing
during the CMS magnet standard ramps down are measured with six 16-bit DAQ
modules. The off-line integration of the induced voltages reconstructs the
magnetic flux density in the yoke steel blocks at the operational magnet
current of 18.164 kA. The results of the flux loop measurements during three
magnet ramps down are presented and discussed.
