Image: Daniels and Daniels
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