system may additionally comprise composition regulating apparatus such as an automatic viscosity control system to restore carrier fluid lost to evaporation or other causes from the mr fluid an automatic viscosity control system 54 may be used to maintain the mr fluid at a constant viscosity by automatically dripping carrier fluid into the reservoir to replace losses the viscosity control system may comprise a viscosity monitoring apparatus functionally connected to a carrier fluid pump 44 with a reservoir of carrier fluid 56 in an exemplary apparatus shown in fig7 viscosity is monitored by use of one or more pressure probes 42 located in the delivery line since pressure changes in the delivery line between the delivery pump s and the delivery nozzle are proportional to viscosity changes for a constant flow rate preferable pressure probes are diaphragm sensors such as cooper pfd 102 since they minimize stagnation points in the line which would allow the mr fluid to settle and clog the sensors the pressure probe signal or when multiple probes are used the difference in the signals from successive pressure probes is proportional to the viscosity of the mr fluid if the flow rate of the fluid is constant the pressure probe signal or differential signal is compared to a reference value and if the signal exceeds the reference an error signal is sent to an electrical relay or motor driver which activates the carrier fluid mini pump 44 until the signal returns below the maximum level in some embodiments the constant of proportionality between the pressure probe signal or differential signal and the error signal must be chosen to avoid over correction resulting in oscillation or under correction resulting in sluggish control of viscosity alternatively the.