The Edison Effect
|
| In early 1880,
Edison and his team were hard at work trying to find a light bulb
filament that worked well. He had already settled on a carbonized
(burned) bamboo filament, but even this solution was not perfect.
After glowing for a few hours, carbon from the filament would be
deposited on the inside walls of the bulb, turning it black. This
would not do.
| 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. |
Edison tried to understand what was happening. His assistant
noticed that the carbon seemed to be coming from the end of the
filament that was attached to the power supply, and seemed to be
flying through the vacuum onto the walls of the bulb.
Edison determined that not only was carbon flying through the
vacuum, but that it carried a charge. That is, electricity was
flowing not only through the filament but also through the evacuated
bulb. In order to measure this flow, he made a special bulb with a
third electrode, to which he could attach an instrument to measure
the current. He reasoned that if the current would flow between the
two ends of the filament, it would also flow to this third
electrode. |
|
|