High Quality Content by WIKIPEDIA articles! A magnetorheological fluid (MR fluid) is a type of smart fluid in a carrier fluid, usually a type of oil. When subjected to a magnetic field, the fluid greatly increases its apparent viscosity, to the point of becoming a viscoelastic solid. Importantly, the yield stress of the fluid when in its active ("on") state can be controlled very accurately by varying the magnetic field intensity. The upshot of this is that the fluid`s ability to transmit force can be controlled with an electromagnet, which gives rise to its many possible control-based applications.