Electrons
Matter is made up of atoms, each consisting of electrically charged parts: a central nucleus, charged positively, surrounded by one or more negative electrons. The nucleus contains most of the mass, whereas the electrons are lightweight, nimble and relatively easy to separate from the rest of the atom. A glowing wire, for instance, emits electrons and can serve as an electron source for the beam used in TV tubes and computer monitors.
Electrons are also most useful in encoding and processing information electrically, a field known as electronics. Nowadays this usually involves transistors, where electrons are loosely held inside a semiconducting material; but at one time all electronic devices--radios, TVs, even early computers--relied on vacuum tubes, in which a hot wire emitted electrons and an arrangement of electrified grids and coils controlled their motion.
Short electromagnetic waves carry enough energy to eject electrons from matter, in particular ultra-violet light and x-rays. A near-vacuum is necessary for any such procedure to be effective, because in ordinary air free electrons collide with molecules, lose their energy and are recaptured. In most of space however matter is so rarefied and encounters are so few that free electrons persist for a long time.
As we climb upwards through the atmosphere, space conditions begin at about 70 km or 45 miles, where electrons liberated by sunlight last long enough to allow air to conduct electricity to a significant degree. That is the beginning of the ionosphere, a layer with enough free electrons (and ions) to play an important role in radio communications. At sunset the electrons of the lowest part of the ionosphere are quickly recaptured and that layer disappears. However, at about 200 km (120 miles), where the density of free electrons is the greatest (up to a million in each cubic centimeter), collisions are so few that the ionosphere persists day and night.
Matter consists of atoms, and atoms consist of electrically charged components—lightweight negative electrons, and positive nuclei.
How do we know?
 One clue comes from the “Edison effect,” discovered by Thomas Alva Edison. Imagine a glass bulb from which air has been pumped, until hardly any of it remains. In one end we embed a metal coil of wire (like that of a flashlight bulb) in the other a metal plate, as drawn. Connect now a battery between the coil and the plate, so that the former is negative and the latter is positive.
No current will flow in this circuit: some atoms or molecules may be left inside the bulb, but they are electrically neutral, and can carry no electric current. Air is an excellent insulator: electric companies can string power lines in the open air and never have to worry about currents dribbling out on their way from the power station to consumers.
Now connect a second battery to the end of the coil, so that a current flows through the coil and heats it up. As the wire begins to glow, a current begins to flow, because now negatively charged particles are emitted from the hot wire, are attracted to the positive charge on the plate and by doing so, complete the electrical circuit.
Suppose the connections of the first battery are reversed, so that now the coil is positive and the plate is negative. Then no current flows, showing that the hot wire releases only negative particles, not positive ones. These particles were named electrons
In laboratory experiments these particles were directed by electrically charged structures (similar to the “electron guns” inside TV picture tubes) to form beams. Those beams were then bent by magnets and by electrified plates, and their behavior was studied. From such experiments and others the mass of the emitted particles, which became known as “electrons”, could be determined. It turned out that they were rather lightweight. The simplest atom, that of hydrogen, contains a central positive particle, a proton, and a single electron, and the proton is nearly 2000 times heavier.
Light, like heat, can also knock electrons out of a metal. If the heated coil in the drawing is replaced by a clean metal plate, and light shines onto it, electrons are again released, and current will flow in the circuit. The explanation of this phenomena, called the photoelectric effect, earned Albert Einstein the 1921 Nobel Prize.
The same process will charge a spacecraft orbiting in the sunlight positively, to a few volts. Sunlight knocks out electrons from the surface and a few manage to escape, leaving the spacecraft positively charged; the situation then stabilizes, because the positive charge prevents any more electrons from leaving.
The word "elektron" in Greek means amber, the yellow fossilized resin of evergreen trees, a "natural plastic material" already known to the ancient Greeks. It was known that when amber was rubbed with dry cloth--producing what now one would call static electricity--it could attract light objects, such as bits of paper.
*Material on this web page courtesy of NASA Goddard Space Flight Center
Authors and Curators:
David P. Stern - NASA/GSFC Code 695 (u5dps@lepvax.gsfc.nasa.gov)
Mauricio Peredo - Raytheon STX Corporation (peredo@istp1.gsfc.nasa.gov)
Last updated March 13, 1999
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