The Amateur Scientist; September 1999; Scientific American Magazine; by Carlson; 2 Page(s)

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.