The Electric Field Sensor plugs into your multimeter or MS120 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 MS120 single axis AC Gaussmeter which functions as the bottom half of the antenna.