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.