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The Edison Richardson Thermal Conduction Valve Diode Effect

OddMix.com - Power Technical Note - TN0904 - by Karl Nagy

Thomas A. Edison spent a lot of his time to perfect a commercially successful incandescent light bulb. In his work with his associates they tried thousands of different potential filament materials. Over a thousand different bamboo's types were obtained, processed and tested. Edison dispatched people into the jungles of Asia to look for new untried bamboo types. One hundred hours was his design goal for bulb life. Eventually they selected the carbon filament. Many side benefits resulted of this research.

Figure 1. The Edison Diode Without a Plate Battery [4 KB]
Figure 1. The Edison Diode Without a Plate Battery

Since availability of electricity is essential for electrical lighting, sources of electric power development progressed rapidly. The developers designed many different dynamos and generators. Edison designed generators were 50% more efficient at the time than those were available. Not only the power supplies needed development but an entire electrical distribution system had to be designed. One of a simpler but just as important part of the light bulb design was the switch, fuse, socket, and lamp base. The last two are still named after the inventor.

Figure 2. The Edison Diode Without a Plate Battery [4 KB]
Figure 2. The Edison Diode With a Plate Battery

Edison also invented the Edison battery that is a made with nickel and iron plates and used diluted KOH for electrolyte. Even the electrodes required special processes and machinery, also invented for the occasion to perforate the holders in a proper way. The perforated sheet metal was formed into pockets and the inside of them had the finely granular, powdered, active electrode material. The Edison alkaline battery is superior to the lead acid in many ways and it is still used to a limited extent in many mission critical applications.

Figure 3. The Edison Diode's Characteristics [4 KB]
Figure 3. The Edison Diode's Characteristics

As all this development work was progressing, Edison added a little metal plate - marked Anode on the circuit diagrams - into an ordinary light bulb and experimented with it. As expected, since vacuum does not conduct electricity, and the interior space was evacuated, there was no current flow between the internal electrodes. Figure 1. Only after he added another, battery into the anode circuit during his experimentation some small current flow was detected. He discovered that current flowed in the vacuum when the plate was made positive with respect to the filament, but not when negative. Figure 2.

After much more careful examination, they found that this current between the filament and the plate was small, and it increased with increasing plate voltage. After awhile the current flow no longer increased even with larger plate voltages. The characteristic of the device is shown on Figure 3. Edison did not do much with this device named diode; a Latin word based on the number of electrodes inside the glass envelope or with the new effect he observed. The original early vacuum tube diodes all used direct filaments. Much later during the Vacuum Tube Era indirect heater filament systems were devised and perfected.


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