Image: Daniels and Daniels ATMOSPHERIC ION COUNTER relies on a fan to draw air through the instrument. A thin brass plate inside the conducting rectangular tube then detects either positive or negative ions in the airflow.

Replenished by the thousands of thunderstorms that constantly pummel our planet, the earth's electric charge produces an electric field that is typically around 100 volts per meter of elevation and that can surge to thousands of volts per meter when a thundercloud rolls overhead. In my July column I explained how to measure these fields with a delightful instrument called a field mill. I also mentioned that we would all be electrocuted instantly were it not for the fact that the atmosphere contains very little free charge (ions and unattached electrons), and so these large fields simply cannot generate dangerous currents. In this issue I thought I would show you how to measure the density of these charges.

Every fraction of a second, cosmic rays strip electrons from some of the normally neutral molecules in our atmosphere. Ionization is also triggered by ultraviolet light, fires and the radioactive decay of certain elements. These processes leave some air molecules positively charged while simultaneously creating a diffuse mist of electrons, some of which are picked up by other atoms. The atmosphere thus contains both positively and negatively charged ions. These particles are extremely scarce. Of the 2.5 ¿ 1019 molecules that reside in each cubic centimeter of air inside your home, only a scant 200 carry an excess negative charge, whereas 250 are positively charged. (The concentrations are often higher outside.) Nevertheless, an instrument that can be built for under $60 readily detects that tiny number. The homemade device, which owes its origin to Bill Lee of AlphaLab in Salt Lake City, is a simplified version of a sophisticated commercial unit AlphaLab (801-487-9492) sells for about $580.

The instrument uses a small fan, like the ones used to cool personal computers, to draw air through a conducting rectangular tube. Inside the tube the air travels over a thin brass plate that is fixed at ground while the tube's electrical potential is set to either +5 or -5 volts. The former repels positive ions toward the central plate, where they pick up electrons and are neutralized. The latter drives negatively charged ions to the plate, where they deposit their excess electrons. Either action causes a current to flow through a resistor, creating a proportional voltage drop that a simple circuit can amplify for you to measure.

You can construct the conducting rectangular tube, measuring 7.6 by 7.6 by 1 centimeters, from four plates of double-sided copper-clad circuit board. The inner surfaces should all be connected electrically so that they can be energized with either +5 or -5 volts. The topmost outer surface must be held at ground, however, and needs to be isolated electrically.

Image: Laurie Grace ELECTRONIC CIRCUITRY for the ion counter must be able to detect a minute current, just 10-15 amp. To do so, a 10-gigaohm resistor forces the tiny signal to generate about 10 microvolts, which a high-impedance operational amplifier can then magnify.

For the brass sensor plate, almost any hobby shop sells sheets of the alloy that can be cut. You'll need a 6.4-centimeter square, but purchase some extra to construct a small wind guard for outdoor operation of your instrument. To prevent stray electric fields from ruining your measurements, cover the downstream opening with an aluminum screen that is electrically connected with the inside of the tube. Note that just a 9-volt battery runs the 12-volt fan. The signal current will be fantastically small--the ions in the air inside your home might generate only 10-15 amps. (Outside sources should produce a somewhat larger current.) To become a detectable signal, such a tiny current must pass through a huge resistance--in our case, about 10 gigaohms. Ohmcraft in Honeoye Falls, N.Y., sells such monster resistors for about $10 each

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