In the 1800's it was noticed that electrical currents and spark discharges caused various effects on objects at a distance. Nearby gas flames would flare and dim, dust particles would clump together, and sparks would jump between closely-spaced objects. In 1865 James Clerk Maxwell proposed that these phenomena were results of electromagnetic waves that emanated from the electrical source. In 1887 Heinrich Hertz demonstrated the existence of these waves & greatly improved the equipment for their generation and detection. In 1892 Sir William Crookes suggested that these waves could be used for communications purposes by sending signals in Morse code in the form of a wireless telegraph system. During the 1890's Guglielmo Marconi worked toward this end and by 1899 he had developed the first practical transmitters and receivers for radio communications.
A giant step forward occurred in 1906 with the invention of the vacuum tube by Lee DeForrest. Tubes could amplify the received signals, greatly increasing the sensitivity of radio receivers. Tube-type transmitters & receivers also made possible voice trans- mission. Previous equipment was incapable of producing voice transmission of adequate quality and radio communication had been in the form of the dot-and-dash Morse code.
At this time radio equipment was used mainly for specialized purposes such as military and ship communications and telegraph services. In 1916 David Sarnoff proposed that radio sets be developed for home use, bring music, lectures, sports events, news and other entertainment. Efforts were now directed toward manufacturing radio sets for the home.
In 1921 the presidential election results were broadcast to the general public. This was the first large-scaled public broadcast to be made. It caused a sensation & interest in radio soared. Many manufacturers now began to make radios for home use and the number of radio sets & broadcasting stations increased rapidly.
The late 1920's and early 1930's saw rapid development in radio technology. Until about 1926 radio receivers required batteries for operation, but around 1927 tubes that could operate on AC current were developed. Around 1930 screen-grid tubes (tetrodes & pentodes) were developed. These tubes had much greater amplifying power than the previous types and they greatly reduced feedback which had caused squealing and howling in the early sets.
In the early 1930's the superhetrodyne receiver was introduced. In this type of set signals tuned in are converted to a single, lower frequency before amplification & detection. The superhet was superior to all previous types in sensitivity, selectivity, and stability. It quickly replaced all other types of sets. Also in the early 1930's the AVC circuit was adopted. With this circuit a voltage proportional to the strength for the incoming signal was used to control the sensitivity for the set so that strong or weak signals gave the same volume of sound. This eliminated the tedious readjustment of the volume control for each station tuned in.
By the mid-30's the home radio set had been developed to nearly
it's modern day form. The only other major change came in the mid- 1950's
with the replacement of vacuum tubes by transistors.
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