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Abstract

A new electrical insulating oil composition having improved low temperature characteristics which composition comprises a mixture of 40% by weight or more of benzyltoluene and the remainder of alkyl substituted diphenylmethane having 15 to 17 carbon atoms which is represented by the following general formula: ##STR1## wherein R.sub.1 and R.sub.2 are C.sub.1 to C.sub.4 alkyl groups and the total number of carbon atoms in R.sub.1 and R.sub.2 is 1 to 4, except that R.sub.1 and R.sub.2 are simultaneously methyl groups; and the proportion of the total quantity of solid phase calculated according to the following general solid-liquid equilibrium equation is 45% by weight or less relative to the total quantity of said composition at -40.degree. C.: ##EQU1## wherein X.sub.i is the equilibrium mole fraction of a component i, in the liquid phase, .DELTA.H.sub.i.sup.f is the heat of fusion (cal.mol.sup.-1), T.sub.i.sup.f is the melting point (K), T is the temperature (K) of the system, and R is the gas constant (cal.mol.sup.-1.K.sup.-1). Electrical capacitors prepared with these electrical insulating oil compositions provide excellent operation at temperatures as low as -40.degree. C., preferably as low as -50.degree. C.

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