The OPAL magnet consists of a water cooled solenoid and an iron yoke to provide flux return. The solenoid and pressure vessel surround the central tracking chambers and the iron yoke is incorporated into the hadron calorimeter.
The purpose of the magnet is to cause charged particles to move in a helical path, therefore aiding measurement of particle momentum within CJ. The solenoid provides a field of 0.435 T within the central tracking region and is uniform to within 0.5%.