The fluid coupling is a type of "soft start" clutch that is often overlooked. It is capable of enduring long periods of high slip and can absorb shock and vibrations. At full load, they can be up to 99% efficient. Torque transmitted increases with decreasing slip.
Eddy current and hysteresis clutches also have no direct connection between the drive and driven shafts. This means that there is no wear between the surfaces transmitting the torque. Hence, life for these devices is long. The eddy current clutch transmits torque as it slips. The hysteresis clutch transmits torque without slip within its load limit. Both types can absorb shock and vibration.
The magnetic particle, powder, or magnetic fluid clutch is a less common device. It consists of a pair of concentric cylinders with a magnetic medium sandwiched between them. When subjected to a magnetic field the magnetic medium links the inner and outer cylinders thereby allowing torque to be transmitted. The relationship between applied current and torque is proportional and the response is fast making this clutch perfect for control applications.