The performance of a surface electric field strength meter, utilizing a broad unfocused beam of electrons, has been evaluated analytically and experimentally. The meter is capable of measuringEfields up to 100 V/cm and possesses a sensitivity of ∼0.1 V/cm in the field regime nearE=0. The meter response to time varying electric fields is limited by the finite transit time (<10−7sec) of the electrons from the source to the detector plates. Simple formulae for the errors introduced in the measurement of time dependent electric fields by finite transit time effects and by displacement currents are presented and indicate errors of ∼0.1 V/cm forEfields rising at 106V/cm sec. Sensitivity of the meter to magnetic field perturbations along three mutually perpendicular directions has been evaluated. The meter response has been examined in a plasma flow simulating spacecraft through the ionosphere.