The electrical and optical threshold voltages for the twisted nematic field effect have been measured using Schiff base mixtures having &Dgr;&egr; values from 2 to 16. The electrical threshold was measured by capacitance techniques, and was smaller than the optical threshold by a factor of ∼ 1.5. By using response time data to estimate changes in average elastic constants of the mixtures, the expected variation of the electrical threshold with &Dgr;&egr; was found.