The thicknesses of insulation considered necessary in this country for transmission cables have been determined largely by experience with the cables in which the insulation was impregnated with a rosin oil compound and had high dielectric loss. If the Rules in the Standards of the A. I. E. E. are a sufficient criterion of the quality of the cable, then the thicknesses of insulation ordinarily used for transmission cables in this country can be very materially reduced, as cables with the present thicknesses of insulation with material and workmanship of the first quality, will pass the tests prescribed by the Standards of the A. I.E. E. with a wide margin frequently exceeding 100 per cent.