The resistance of the PTC thermistor is composed of the grain resistance and the grain boundary transition resistance. Particularly in the hot state, the strong potential barriers are determining resistance. Higher voltages applied to the PTC thermistor therefore drop primarily at the grain boundaries with the result that the high field strengths dominating here produce a break-up of the potential barriers and thus a lower resistance. The stronger the potential barriers are, the greater is the influence of this "varistor effect" on resistance. Below the reference temperature, where the junctions are not so marked, most of the applied voltage is absorbed by the grain resistance. Thus the field strength at the grain boundaries decreases and the varistor effect is quite weak.
Fig 3 shows the typical dependence of resistance on field strength. It can be seen that the difference in resistance is largest between R(E1), R(E2) and R(E3) at temperature Tmax and thus in the region of maximum resistance.