Atmospheric electrostatics. wahlin l. chapter 6

CHAPTER 6
INSTRUMENTATION AND MEASUREMENTS
6.1 MEASUREMENTS
Atmospheric electricity is a field that is very easy to get into because it
Does not require a large capital investment for measuring equipment.
It is a difficult field, however, when it comes to the understanding and
Interpretation of the various measurements. For example, despite vast
Data collected over two centuries, there is still no agreement on how
Thunderclouds charge or how the earth-atmosphere fairweather field is
Maintained.
Today’s advanced technology offers eloquent and accurate
Instruments with fast response time. It is now possible to construct
Simple electrometers and electromagnetic radiation detectors using
Chip circuitry. Video recording equipment is becoming a household
Item that also has great potential for lightning research, making it
Possible to view flights of lightning bolts in micro-second intervals.
6.2 ELECTRIC FIELD MEASUREMENTS
Most common are measurements of the electric field at or near the
Earth’s surface. The potential gradient at the earth’s surface varies in
Many ways but usually averages about +100 volts per metre during
100 ATMOSPHERIC ELECTROSTATICS
Fine weather in the absence of clouds. Except for the normal diurnal
Variations of the fairweather field (see sec. 2.2), strong rapid changes
Are often registered which might be due to electrically charged dust or
Heavy pollution. Marked changes in the fairweather field have been
Observed before the onset of fog or during temperature inversions in
The lower atmosphere. Many of the above field effects and their causes
Are still not fully understood. The electric field below thunderclouds
Are usually very strong and easy to detect. Rapid field changes occur
During lightning discharges and field reversals are common. Field
Measurements below thunderclouds reveal such features as charging
Rates, electric polarity and discharge rates to
Fig. 43 Different antenna configurations for electric field measurements in
The atmosphere. (a) Disc antenna. (b) Whip antenna. (c) Whip antenna with
Radioactive probe. (d) Long wire antenna.
Mention a few. There are several ways one can measure the electric
Field in the atmosphere. A simple approach is to connect an
Electrometer to an antenna of some sort such as shown in Fig. 43. The
Antenna can be a conducting disc placed at a predetermined height
INSTRUMENTATION AND MEASUREMENTS 101
Above ground (Fig. 43a). The disc will charge and reach a potential
Which is equal to or very near the potential of the atmosphere at that
Height. The diagram in Fig. 43b shows an electrometer connected to
A short whip antenna which gives voltage readings that are difficult to
Calibrate since the antenna whip will protrude through many levels of
Electric potential. An ion-producing radioactive alpha source at the
Tip of the whip antenna, see Fig. 43c, will increase the electric
Conductivity in the air near the tip and ensure a better accuracy of
Potential measurement as a function of height. A long wire,
Suspended above ground at predetermined levels, will give accurate
Readings of atmospheric potentials as a function of height, see Fig. 43d.
6.3 THE ELECTROMETER
There are many excellent electrometers commercially available, but
Since the electronic market of today can offer a variety of sophisticated
Integrated circuits, it makes simple home made devices nearly as
Fig. 44 Electrometer circuit.
102 ATMOSPHERIC ELECTROSTATICS
Effective. For example, the circuit in Fig. 44 has been used by the



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Atmospheric electrostatics. wahlin l. chapter 6