the relationship between the electrical field strength and the yield strength is linear after the electric field reaches 1 kV/mm. The GER is a high yield strength, but low electrical field strength and low current density fluid compared to many other ER fluids. The procedure for preparation of the suspension is given in. The major concern is the use of oxalic acid for the preparation of the particles as it is a strong organic acid. Applications The normal application of ER fluids is in fast acting hydraulic valves and clutches, with the separation between plates being in the order of 1 mm and the applied potential being in the order of 1 kV. In simple terms, when the electric field is applied, an ER hydraulic valve is shut or the plates of an ER clutch are locked together, when the electric field is removed the ER hydraulic valve is open or the clutch plates are disengaged. Other common applications are in ER brakes (think of a brake as a clutch with one side fixed) and shock absorbers (which can be thought of as closed hydraulic systems where the shock is used to try to pump fluid through a valve). There are many novel uses for these fluids. Potential uses are in accurate abrasive polishing and as haptic controllers and tactile displays. ER fluid has also been proposed to have potential applications in flexible electronics, with the fluid incorporated in elements such as rollable screens and keypads, in which the viscosity-changing qualities of the fluid allowing the rollable elements to become rigid for use, and flexible to roll and retract for storing when not in use. Motorola filed a patent application for mobile device applications in 2006. Problems and advantages A major problem is that ER fluids are suspensions, hence in time they tend to settle out, so advanced ER fluids tackle this problem by means such as matching the densities of the solid and liquid components, or by using nanoparticles, which brings ER fluids into line with the development of magnetorheological fluids. Another problem is that the breakdown voltage of air is ~ 3 kV/mm, which is near the electric field needed for ER devices to operate. An advantage is that an ER device can control considerably more mechanical power than the electrical power used to control the effect, i.e. it can act as a power amplifier. But the main