Edison Effect

In early 1880, Thomas 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. 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.

While he was proven to be right about the flow, Edison could not explain it, and the third electrode did not prevent blackening of the bulb, so he moved on to other experiments. But he did patent the new device, because he believed that it might have some commercial applications, such as measuring electric current. Although he did not realize it, Edison had discovered the basis of the electron tube (also called a vacuum tube). Many years later, modified light bulbs would be used not to make light, but to control a flow of electrons through a vacuum. The electron tube would become the basis of modern electronics. Years later, when he was elderly, the discovery of what became known as the “Edison Effect” was remembered, but because Edison had no idea what it was or how it worked, he is rarely given credit for this contribution to the development of electronics.

Vacuum Tube

The simplest vacuum tube, the diode, contains only a heater, a heated electron-emitting cathode (the filament itself acts as the cathode in some diodes), and a plate (anode). Current can only flow in one direction through the device between the two electrodes, as electrons emitted by the cathode travel through the tube and are collected by the anode. Adding one or more control grids within the tube allows the current between the cathode and anode to be controlled by the voltage on the grid or grids. Tubes with grids can be used for many purposes, including amplification, rectification, switching, oscillation, and display.

The earliest vacuum tubes evolved from incandescent light bulbs, containing a filament sealed in an evacuated glass envelope. When hot, the filament releases electrons into the vacuum, a process called thermionic emission, originally known as the "Edison Effect". A second electrode, the anode or plate, will attract those electrons if it is at a more positive voltage. The result is a net flow of electrons from the filament to plate. However, electrons cannot flow in the reverse direction because the plate is not heated and does not emit electrons. The filament (cathode) has a dual function: it emits electrons when heated; and, together with the plate, it creates an electric field due to the potential difference between them. Such a tube with only two electrodes is termed a diode, and is used for rectification. Since current can only pass in one direction, such a diode (or rectifier) will convert alternating current (AC) to pulsating DC. Diodes can therefore be used in a DC power supply, as a demodulator of amplitude modulated (AM) radio signals and for similar functions.

          

                                                                                                                                                                                                   triode

           diode                   

Early tubes used the filament as the cathode, this is called a "directly heated" tube. Most modern tubes are "indirectly heated" by a "heater" element inside a metal tube that is the cathode. The heater is electrically isolated from the surrounding cathode and simply serves to heat the cathode sufficiently for thermionic emission of electrons. The electrical isolation allows all the tubes' heaters to be supplied from a common circuit (which can be AC without inducing hum) while allowing the cathodes in different tubes to operate at different voltages. H. J. Round invented the indirectly heated tube around 1913.

The filaments require constant and often considerable power, even when amplifying signals at the microwatt level. Power is also dissipated when the electrons from the cathode slam into the anode (plate) and heat it; this can occur even in an idle amplifier due to quiescent currents necessary to ensure linearity and low distortion. In a power amplifier, this heating can be considerable and can destroy the tube if driven beyond its safe limits. Since the tube contains a vacuum, the anodes in most small and medium power tubes are cooled by radiation through the glass envelope. In some special high-power applications, the anode forms part of the vacuum envelope to conduct heat to an external heat sink, usually cooled by a blower, or water-jacket.