  The principal difficulty in large magnetic systems having an extensive flux
return yoke is to characterize the magnetic flux distribution in the yoke steel
blocks. Continuous measurements of the magnetic flux density in the return yoke
are not possible and the usual practice uses software modelling of the magnetic
system with special 3D computer programs. The flux return yoke of the Compact
Muon Solenoid (CMS) magnet encloses a 3.8 T superconducting solenoid with a
6-m-diameter by 12.5-m-length free bore and consists of five dodecagonal
three-layered barrel wheels around the coil and four endcap disks at each end.
The yoke steel blocks serve as the absorber plates of the muon detection
system. A TOSCA 3D model of the CMS magnet has been developed to describe the
magnetic field outside of the solenoid volume, which was measured with a
field-mapping machine. To verify the magnetic flux distribution calculated in
the yoke steel blocks, direct measurements of the magnetic flux density with 22
flux loops installed in selected regions of the yoke were performed during the
CMS magnet test in 2006 when four "fast" discharges of the CMS coil were
triggered manually to test the magnet protection system. No fast discharge of
the CMS magnet from its operational current of 18.2 kA, which corresponds to a
central magnetic flux density of 3.8 T, has been performed that time. For the
first time, in this paper we present measurements of the magnetic flux density
in the steel blocks of the return yoke based on the several standard linear
discharges of the CMS magnet from the operational magnet current of 18.2 kA. To
provide these measurements, the voltages induced in the flux loops have been
measured with six 16-bit DAQ modules and integrated offline over time. The
results of the measurements during magnet linear ramps performed with a current
rate as low as 1-1.5 A/s are presented and discussed.
