Electric Field Mills

Since the triggered-lightning threat to launch vehicles depends on the strength of atmospheric electric fields in and around clouds, an awareness of these fields along the intended flight path is important for preventing a lightning strike during a vehicle's ascent. Unfortunately, electric fields can be neither seen nor easily measured.

The device normally used to measure atmospheric electric fields is a type of sensor called an electric field mill. It can be used on the ground to gain information about the field aloft, or it can be flown on an airplane or balloon. Thirty-one ground-based field mills at Cape Canaveral Air Force Station and the Kennedy Space Center are used to measure the electric field just above the surface of Earth, in the lowest meter of the atmosphere, and determine if it meets the safety levels specified in the Lightning Launch Commit Criteria.

The small, cylindrical field mill is mounted upside down. Inside it are a rotating metallic shutter that is grounded and a set of metal stators that are insulated from ground and that are alternately covered and uncovered by the rotor. When the rotor uncovers the stators, they are exposed to the atmospheric field and a surface charge appears on them. When the rotor covers the stators, the surface charge disappears. The magnitude and polarity of the ambient electric field can be determined by measuring the amplitude and phase of the current that flows to and from the stators.

The Amateur Scientist column in the July 1999 Scientific American provided details on creating a homemade field mill using trash cans, cake pans, an electric motor, and other miscellaneous hardware.

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