The ionised layers of the atmosphere from about 40 to 200 kilometers up are positively charged with respect to the earth. There is thus an electric field extending from this layer down to the earth.
Nearer the earth there are clouds just a few hundred feet up. These clouds frequently have a negative charge at the bottom and positive at the top. With the advent of an electric storm these charges are sufficiently large to cause a breakdown in our atmosphere producing the familiar thunder and lightning.
Added to these natural phenomena are the man-made ones, generated for example by switching transients in our mains supply system.
These various forms of atmospheric electricity are difficult to detect because the air is a very poor conductor and consequently highly specialised and very sensitive instruments are needed to detect them.
I have spent many happy hours designing and building the necessary instruments and using them to record the presence and changes in our electric atmosphere.
The instruments generaly take one of two forms. A very high input resistance voltmeter (HRVM) or a very sensitive ammeter. An example of the former was published in Electronics Today International (otherwise know as ETI!) in March and April of 1992. Whilst one of the latter was shown in the Maplin Magazine Electronics and Beyond in September and October of 1997.
The simplest way to obtain some useful figures about the atmosphere and its electrical content is to support a metal plate above ground on an insulator and then treat the plate as though it were a battery. Going back to school days one may recall that a battery possesses both voltage and internal resistance and both of these can be measured.
Recent improvements in components has made this all much easier and suitable equipment is described in the following.
The sketch below illustrates the arrangement.
The plate is made from 1.2 to 1.6 mm aluminium sheet. 0.1 sq M (316 mm square) is a convenient size. If the edges of the plate are dressed down slightly as suggested in the sketch this will stiffen it and make it less succeptible to wind. The support is best made from plastic piping such as 41mm O/D domestic waste pipe.
The meter is represented as a perfect voltmeter in parallel with a resistor. One side connected to the plate, the other to a variable high voltage power supply (HVPS in the sketch). The other terminal of the power supply is earthed.
There is a difficulty with this arrangement in that the voltmeter has
active components and therefore needs a power supply. And this has to be able
to be raised by the HVPS to a voltage above ground. However this difficulty can
be overcome. A suitable power supply is described along with the voltmeter
giving component values and suggested costructional details. (See bottom of
page for meter, HVPS, interconnections and display using moving-coil meters).
For first attempts fair weather conditions are best. A blue sky, and very little or no wind.
Set the system up with the plate supported 0.5 metre above ground.
With the HVPS set to zero the meter will indicate any voltage present at the plate. Assuming a positive voltage is present (and it usualy is in fair weather conditions) note the reading which we will call V and then raise the voltage of the HVPS until the meter indicates zero. At this point the voltage provided by the HVPS will be the same as that on the plate. Note the voltage which we will call E. Anything from zero to more than 200v is not uncommon at 0.5 M.
Referring to the sketch again, as the plate resistance and the meter resistance are in series the same current flows through each. Consequently the voltages across them are in proportion to the resistance value. This means that -
Resistance of atmosphere/Resistance of meter=(E-V)/V
Resistance of meter=10^11 or 0.1 T ohms
Resistance of atmosphere=0.1*(E-V)/V T ohms
Voltage at plate=E.
In the above the average E is 71.6 volts and the average resistance is 6.9 T Ohms.
As the plate is 0.1 sq metre the figures can be standardised by dividing the resistance by 10 and the value given in ohms per square metre.
ANOTHER POSSIBILTY
The flat plate technique that has just been described gives "fair weather" information related to the electric field. Another interesting approach is to measure the conductivity of the atmosphere at any time, or even continuously using some form of recorder or computerised logging system.
The conduction will depend on the extent to which the air is ionised, and will have two components. Positive ions short of an electron and negative ions having an extra electron.
A device suitable for this approach is describe in the add-on - see below.
High input resistance voltmeter.
Dual power supply.
Connections.
Display.
An add-on to detect ions.
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