We have examined the statistical relationship between solar wind parameters and the polar cap potential (ΦPC) using data from the Wind and ACE satellites and the northern polar cap index (PCN) as a proxy for ΦPC. We found that PCN tended to be saturated when the value of the merging electric field (Em) was high (greater than 5 mV/m). Furthermore, it was found that the degree of saturation does not depend on the intensity of the interplanetary magnetic field but depends on the value of Em. These results are in quite reasonable agreement with the predictions of the model of Hill et al. <1976>. This fact suggests that the development of magnetospheric convection is not controlled by the efficiency of the dayside merging process itself but is controlled by the coupling among the solar wind, the magnetosphere, and the ionosphere.