During periods of substorm activity, midlatitude geomagnetic data often reveal a depression in the northward component of the magnetic field in a limited region near the dusk meridian. This magnetic signature has usually been attributed to a partial ring current which forms as a result of the substorm activity. It is intriguing, however, that not all substorms are associated with the presence of a partial ring current. Results of a study which examines the relationship between substorm activity and the formation of the partial ring current will be presented. Using midlatitude magnetic data, the partial ring current may be parameterized by the maximum strength of the field depression which develops near the dusk meridian. This parameter has been related to time integral of the AE, AU and AL indices as well as to the dawn-to-dusk component of the interplanetary electric field. The results show little relation of the parital ring current parameter to the measures of substorm activity but a high correlation with the interplanetary electric field. An investigation of timing relationships is also reported. A comparison of the onset times of auroral zone activity and partial ring current growth shows that the partial ring current is as likely to begin to develop before the onset of auroral zone activity as after. A southward turning of the interplanetary magnetic field however, consistently precedes the establishment of a partial ring current. These results are interpreted to indicate that enhancement of the fractional solar wind emf applied across the magnetosphere plays a direct causative role in the development of the partial ring current.