The electric strength of silicone liquids

Controlled measurements of the d.c. electric strength of certain pure liquid poly-dimethyl siloxanes or silicones are reported. The liquids tested were the four lower members of the D.C. 200 series of silicone fluids having viscosities of 0.65, 1, 1.5 and 2 centistokes. The measurements were made under conditions identical with those employed for earlier experiments on pure n-paraffins which have a similar molecular structure, so that useful comparisons could be made. The use of a hydrogen-thyratron circuit to by-pass the discharge energy when breakdown occurred was found to be essential, since the discharge products, which probably included silica, were appreciably greater than for the n-paraffins. Like that of the n-paraffins, the strength of the silicones, which is of the order of 900kV/cm, was found to increase with chain length and with decreasing test-gap. The liquid of 0.65 centistokes viscosity was also tested over the temperature range ¿55°C to 85°C, and it was found that the strength increased with decreasing temperature, the increase being most marked at temperatures below about 20°C. This behaviour is similar to that of the n-paraffins already tested and cannot be ascribed to density changes alone.