When a magnetic field is applied by a coil to the powder, it forms chains connecting the disc and rotor.
Magnetic particle clutches have made equipment control easier while also prolonging life of machine components.
In the late 1970s and early '80s, these were used in high-speed line printers to stop a spinning drum stamped with 96 or more columns, and 40 or more rows containing the alphabet.
The drum was stopped momentarily by the clutch while hammers struck ink and paper in the appropriate column.
Paper mills require material to be fed into their rollers at a constant rate.