"In a hysteresis coupling, one side is a ring or disc shaped piece made from a permanent magnetic material that is ferromagnetic and thus able to undergo hysteresis. The other side of the coupling is then made of an electrical conductor back with iron, again usually in a disc or ring shape. The ability of the coupling to “remember” magnetic fields and because they start one way, reverse, and then reverse again in a cycle means that hysteresis couplings “slide” smoothly, that is, they can take up motion in the system, and heat up less than other designs. Magnetic hysteresis couplings can be accurately set to +/- 5% for smooth and friction free control of torque. And they are designed to deliver accurate and friction free tension control for bottle filling, textile, thin film and labeling applications."