Recently we reported that the viscosity of p-methoxybenzylidene-p'-n-butylaniline (MBBA) in the presence of electroconvection shows a characteristic decrease in the high-voltage regime. The decrease in viscosity seems to be attributed to the negative contribution of electric stress. To verify the conjecture, we have estimated the magnitude of the electric stress by the response of shear stress when an ac sinusoidal voltage was turned on and off. Then, it is found that the electric stress is negative and does not depend on the shear rate. Under sufficiently low shear rate, the viscosity becomes negative due to the electric stress.
