Like electrical charges repel and unlike charges attract. An electric force field, referred to as an electric field, is said to exist around electrical charges. Electric field defines the force per charge (E = F/q), and has both a magnitude and direction (vector quantity). The units of electric field are Newtons/Coulomb, which is equivalent to the more commonly expressed units of Volts/meter (V/m).

Atmospheric electric fields have been studied for some time, primarily because of interest in cloud electrification and lightning warning. At the surface of the earth, large magnitude electric fields occur during local thunderstorms. Hence, atmospheric electric field is useful in assessing the local lightning hazard.

Atmospheric electric fields have been measured for decades by electric field meters dubbed “field mills”. Traditional field mills employ a spinning metal rotor (vane) electrically connected to Earth ground potential. The grounded spinning rotor alternately shields and exposes sense electrodes from the electric field to be measured, resulting in a modulation of induced electrical charge. Charge amplifiers convert the modulated charge into voltages that are proportional to the low-frequency £ electric field.

Unlike traditional rotating vane field mills, the CS110 utilizes a reciprocating shutter. A stepper motor opens and then closes the reciprocating shutter by 45° during measurements. The reciprocating shutter is electrically connected to Earth ground potential by a flexible stainless-steel strap operated below its fatigue limit, resulting in an ultra-reliable electrical ground connection.

The reciprocating approach also offers improved low-frequency error performance, as compared with traditional rotating vane field mills, because of the availability of a convenient zero-field (closed shutter) reference. This zero-field reference is utilized by the CS110 to measure and then correct for electronic offset voltages, contact potentials and leakage currents of each individual measurement (Patent Pending).