The Electric Field Sensor plugs into your multimeter or MSI-95 gaussmeter to measure the electric field strength. Calibration is for vertical  polarization at 60 Hz. Accuracy is around +/-20% since an E-field measurement is strongly affected by the surroundings (earth, measurer's body, metal objects, etc).

It is most accurate when used with the multimeter supplied with the MSI-95 single axis AC Gaussmeter which functions as the bottom half of the antenna.