Joe Seabloom became interested in electrostatic motors after reading the article, Electrostatic Motors are Powered by the Electric Field of the Earth by C. L. Stong in “The Amateur Scientist” column of the October 1974 issue of Scientific American. It is known that there is a potential difference of about 400,000 Volts between the ionosphere and the Earth’s surface. As a result, there is an average electrostatic potential gradient near the Earth’s surface of approximately 100-200 V/m. When referenced to ground, the voltage a few meters above the Earth’s surface is quite large, but there is very little current available to power most devices, thus the Earth’s electric field remains an untapped energy source. Electrostatic motors provide one way of tapping this power source. Most motor use high currents and low voltages to create magnetic fields that, in turn, exert torques on magnets within the motor that cause it to spin. In contrast, electrostatic motors use high voltages and low currents to create electric fields that, in turn, exert torques on electrets within the motor that cause it to spin. An electret is the electrostatic equivalent of a magnet: just as a magnet has a permanent magnetic dipole field, an electret has a permanent electrostatic dipole field.
Joe began by creating a prototype electrostatic motor that could be powered using a Wimshurst electrostatic generator.
This protoype consists of two plastic cylinders wrapped with aluminum foil, and a plastic cup balanced on a sharp point. There were three vertically aligned pieces of aluminum foil taped to the cup, and a thin wire protruding from each cylinder. The purpose of the wire is to spray charge onto the cup. In the picture above, the left cylinder is positively charged and the right cylinder is negatively charged. The wire from the left cylinder sprays positive charge onto front of the cup. This causes the front of the cup to become positively charged and is thus attracted to the negatively charged cylinder on the right. In a similar manner, the wire fron the right cylinder sprays negative charge onto the back of the cup, thus causing it to be attracted to the positively charged cylinder on the left.
A more efficient motor can be made using an electret made of wax. Several types of wax are recommended for electrets: a mixture of 75% carnauba wax and 25% beeswax is recommended, but is difficult to remove from the mold. Joe had some success using two pieces of candle wax, with one piece charged positive and one piece charged negative. The final motor is shown below.
The electret disk is sandwiched between plates of copper. Each disk of copper is split in two, thus there are two plates on the left and two plates on the right. The remainder of the apparatus is made of plexiglass which is a durable insulator. A schematic illustration of the motor is shown below.
The motor requires very little current (a few picoamps) but a fairly high voltage (about 5000V) to run. It can therefore be powered by raising a wire “antenna” about 30 meters above the ground using an aluminum coated mylar balloon. The antenna is connected to the motor via fine wire (40 gauge “wire wrap” wire). Note that the wire alone is not very strong and that additional support (e.g. fishing line) is required to prevent the balloon from escaping.
Although this motor may not seem practical, electrostatic motors may well prove to be the most efficient way to make microscopic motors. See this site for some pictures of microscopic electrostatic motors.
Some useful references:
HOW TO MAKE ELECTRETS, DEVICES THAT HOLD ELECTRIC CHARGE. C. L. Stong in “The Amateur Scientist”, Scientific American; July 1968.
ELECTROSTATIC MOTORS ARE POWERED BY ELECTRIC FIELD OF THE EARTH. C. L. Stong in “The Amateur Scientist”, Scientific American; October 1974. Available online at http://f3wm.free.fr/sciences/jefimenko.html
MEASUREMENT OF THE ATMOSPHERIC ELECTROSTATIC POTENTIAL GRADIENT NEAR SEA LEVEL. Mary R. Cummings, Howard W. Nicholson, Jr. and Deborah R. Porto in American Journal of Physics, Vol. 49, No. 12, pages 178-1180; Dec. 1981.
SIMPLE, SENSITIVE VOLTAGE MOTOR USING 2-LITER SODA BOTTLES. William J. Beaty on the Science Hobbyist web site, http://amasci.com/emotor/emotor.html