Abstract: The invention relates to an electrorheological fluid comprising fine silica particles, to which an alcohol-modified silicone oil is bonded, compounded with a nonconducting liquid, to which a polarizing agent is added as necessary. Use of fine silica particles of which the surface is modified by an alcohol-modified silicone oil according to the invention provides an electrorheological fluid which have low initial viscosity, high viscosity increase ratio and excellent dispersion stability.

Abstract: A fluid composition for use in fluid couplings has the superior anti-gelling properties of an organopolysiloxane base oil, is low in changes in viscosity and changes in torque, is stable and has extremely high durability, and is especially useful because of its viscosity as a fluid for "fan" couplings. The fluid composition according to the invention is provided, by adding at least one type of ferrocene derivative, in a weight ratio wherein the atomic iron content is 10 ppm to 5,000 ppm, selected from the group consisting of the compounds represented by the general formula (1) and the general formula (2) for an organopolysiloxane base oil having a viscosity of 50 mm.sup.2 /s to 20,000 mm.sup.2 /s at 25.degree. C. The compounds of general formula (1) may be represented by the following formula: ##STR1## wherein R.sub.1 and R.sub.2 are identical or different, and are each independently selected from the group consisting of C.sub.2-25 alkyl, alkenyl, cyclo-alkyl, and cyclo-alkenyl. Additionally, one of R.sub.