The cross-polar cap potential &PHgr;pc on March 18 and 19, 1978, is determined by the numerical scheme developd by Kamide et al. (1981) on the basis of magnetic records from the six IMS meridian chain stations and others. The potential drop &PHgr;dc thus determined is correlated with the energy coupling function of &egr; and the ΔE index. It is found that &PHgr;pc(kV)=4.2¿10-1&egr; These results are in fair agreement with the satellite results obtained by Reiff et al. (1981). Thus, the ground-based magnetometer inversion technique has been advanced to the point that &PHgr;pc can be inferred from the IMS meridian chain data with fair accuracy.