Abstract: An electro-rheological fluid which comprises a solid particulate substance contained in a hydrophobic vehicle which is liquid at atmospheric pressure at least at temperatures below 50.degree. C. and which comprises a compound of the formula:(X).sub.n --Ar[Q--Z].sub.pwherein:Ar represents an aromatic nucleus;Q represents an oxygen or a sulphur atom, or a group of the formula CY.sub.1 Y.sub.2, SO, SO.sub.2, SiF.sub.2, --OSi(Y.sub.1 Y.sub.2)O-- in which Y.sub.1 and Y.sub.

Abstract: An electroviscous (EV) fluid composition whose fluid properties are altered in the presence of an electric field comprises water-containing particles of a phenol-formaldehyde polymer dispersed in a non-conducting liquid. The composition, selected from the broad range of fluid compositions of UK Pat. No. 1570234, possesses improved fluid properties combined with markedly reduced electrical conductivity, and is thus more suitable for use in applications such as at the interface between electronics and power hydraulics for example.In a preferred embodiment of the present invention the polymer comprises the dilithium salt of 2,2', 4,4' tetrahydroxybenzophenone condensed with formaldehyde, dispersed at 30% V/V in a mixture of brominated diphenyl methane (BDM) liquid components having the averaged formula C.sub.6 H.sub.4 Br.CH.sub.2.C.sub.6 H.sub.5.