The science of electronics can be considered as beginning with the discovery of the “Edison effect.” In 1883 Thomas A. Edison was endeavoring to determine why his incandescent lamps were burning out at the positive end of the filament. (Only direct current was used at the time, as alternating current did not come into common use until a much later date.) In the course of his experiments he sealed a second electrode into some of his experimental lamps and found that, if this second electrode was connected through a galvanometer to the positive end of the filament, current would flow. When the electrode was connected to the negative end of the filament, no current would flow. This effect was the first actual proof that current was the flow of negative charge. Edison, however, was unable to explain it, and since he had more urgent work at hand, he did not continue the investigation.
In 1897 Sir J. J. Thomson of the Cavendish Laboratories definitely established the existence of the electron and explained the Edison effect on this basis. He proved that the electron was a negatively charged particle and that it was the motion of this particle under the influence of electric fields that constituted current flow. He also established the fact that the ratio of charge to mass of the electron was constant for all electrons. In 1910 Dr. Robert Millikan measured the electrostatic charge on the electron and found it to be constant for all electrons. This result, combined with Thomson’s determination of the charge-to-mass ratio, allowed the mass of the electron to be calculated. It also proved that all electrons are identical.
Sir J. A. Fleming, another Englishman, was the first to put the Edison effect to work. In 1904 he patented the first electronic tube, which was called the Fleming valve. This was a simple diode used as a rectifier. However, since alternating current was not in common use at that time, the tube was of little practical value.
Probably the greatest early contribution to the electronic art was made in 1906 by Dr. Lee De Forest, who added a third element, which he called a grid, to the diode. By use of this grid he was able to control relatively large electron currents in the anode circuit by means of varying potentials applied to the grid and with the expenditure of very little power in the grid circuit. Since that time many improvements have been made, so that today we have not only diodes and triodes, but also tetrodes, pentodes, heptodes, and others, each type being named in accordance with the total number of electrodes it possesses. The electronic tube is no longer a laboratory plaything but has advanced to the stage where it is the basis of an entire industry which is rapidly increasing in magnitude.
New and very promising additions to the electronic art have recently become prominent with the development of such devices as improved crystal diodes, thermistors, transistors, and magnetic amplifiers. Today, these devices become as important to the engineer.