Inventors of the thermionic valve

As the telephone line networks in the United States and Europe expanded to include intercity transmission over ever-longer distances, problems of unsatisfactory reception related to attenuation of the line current and poor signal-to-noise ratio began to be apparent. At first these difficulties were fought by using heavier-weight, and therefore lower-attenuation, copper conductors, but this approach became impracticable and uneconomic beyond a certain distance. There was also the problem of poor transmission quality resulting from the greater attenuation of the higher-frequency components of the transmitted voice signals compared with the low frequencies. The ideas initiated by Heaviside on inductive loading of telephone cables, developed by Michael Pupin and by George Campbell of the American Telephone and Telegraph Company, improved transmission quality but left unsolved the problem of overall line loss. Early attempts at a repeater/amplifier involved mechanically coupling an electromagnetic receiver and a carbon granule microphone in a battery circuit—a device that produced amplification but yielded poor quality. It was clear that what was needed was some means of amplifying the voice signals without distorting them.