Measurement of the electrical conductivity of certain high polymer insulating materials

The electrical conductivities of the high polymer materials used in this investigation (polystyrene, methylmethacrylate, and polyvinylbutyral) were so low that special equipment was required for their measurement. A Compton electrometer was used as a null indicator in a circuit similar to that described by P.J. Higgs, Journal of Sci. Instruments, Vol. 10, page 169, June 1933. This circuit is shown in Fig. 1. The electrometer is shorted except during a measurement. The deflection of the electrometer M during measurement is kept at zero by decreasing the capacitance of the variable air capacitor C at such a rate that it nullifies the effect on the electrometer of the current through the specimen. The capacitance is changed at a uniform rate by an electric motor which is geared to the capacitor. The speed of the motor is controlled by a variable series resistance. The conductance (G) of the specimen is computed from the formula: G = E_c/E_x · Δ_c/Δ_t where E_c and E_x are the voltages applied to the variable air capacitor and the specimen respectively, and ΔC is the capacitance change in time Δt. By the use of various voltages and capacitors of different sizes it is possible to measure conductances ranging from 10^-9 mho to 10^-16mho.