It is important to measure electric potential on a surface of plant for analyzing plant physiology or growth of plant. There are many reports that measure electric potential of a plant inserting fine needle electrodes into a plant.
New method to measure electric potential on a surface is developed using the new type electrode system without any disturbing or destroying a plant. This electrode system is constructed by two electrodes and two insulations. When the known AC voltage is given to the electrode 1 of the double layer electrode and the electrode 2 is attached to the surface of a plant beyond the insulating film, the DC voltage and the AC voltage are induced on the electrode 2 according to the potential on the surface of a plant and the AC voltage on the electrode 1. The electric circuit is developed to calculate automatically the potential on the surface of plant from the voltages induced on the electrode 2. As the application of this system, the potential distribution on the surface of a cucumber is measured. The high potential on the surface of the cucumber shows the vivid and rich nourishing part in it.