The dielectric anisotropy &Dgr;&egr; and threshold voltageVthfor electrically controlled birefringence have been measured for a series of nematic liquid crystals (binary mixtures of MBBA and PEBAB) with different negative &Dgr;&egr; values. The threshold voltage varies as |&Dgr;&egr;|−1/2, in agreement with theoretical predictions. The bend elastic constantK33evaluated from these experiments is in order‐of‐magnitude agreement with recent measurements for pure MBBA.