A Brief History of the Events Leading to the Development of the Crystal Radio
The first electrical telegraph systems were developed and placed into operation in the 1830s. The original design required the physical connection of two wires linking the transmitting and receiving terminals in order to pass an electrical signal. Then, in 1837, Carl August von Steinheil, discovered that if he connected each end of one of the two telegraph wires to metal plates and buried the metal plates in the ground he could use the single remaining wire for telegraphic communication. The single wire concept was eventually abandoned in favor of the two wire metallic circuit due to unacceptable interference from electrical currents in the earth. However, the revelation that the ground could be used to transmit and receive electrical signals led to speculation that it might be possible to eliminate both wires in a communications system.
Communication between two distant terminals in early wire telegraph systems was established by the use of Morse code. The operator on the transmit side was only required to turn the signal on or off by use of a ‘key’ and the operator on the receive side detected the signal by the presence or absence of a transmit signal detected by the clicking of an electromagnetic ‘sounder’ which had its signal amplified by a small wooden enclosure called a ‘resonator’. Voice communication was still many years in the future.
By 1874, wire telegraph messaging using Morse code was increasing so rapidly that inventors began experimenting with ways of transmitting multiple messages simultaneously on the same wire lines to avoid the huge expense of constructing more and more telegraph lines. To meet this challenge, Alexander Graham Bell began working on a multi-reed apparatus as a method of sending multiple tones on a telegraph wire. In 1875 Bell realized that his electrical reed apparatus showed promise for transmitting the human voice by wire telegraph. In 1876, Bell received a patent for an apparatus which could transmit the human voice or other sounds telegraphically by causing electrical undulations in the air at the receiving terminal.
While these communications breakthroughs were advancing wire telegraphy, in 1874, Professor Ferdinand Braun, began experimenting with the ‘mysterious and very peculiar electro-magnetic properties’ of crystalline minerals by bringing them into contact with various oxidized metals. He found that certain metal to mineral combinations allowed a greater amount of electric current to flow through the combined materials in one direction rather than the other direction. Then, in 1887, Heinrich Hertz proved the existence of electromagnetic waves by wirelessly transmitting and receiving radio pulses through the air. His transmitter produced an oscillating spark discharge and his receiver signaled the presence of the oscillating discharge by emitting sparks across a gap in the receiver mechanism.
Radio waves travel great distances through the air by means of oscillating electromagnetic fields. When radio waves strike an electrical conductor, such as an Antenna wire, the oscillating electromagnetic fields induce an Alternating Current (AC) in the Receiving conductor. The transmitted information contained in the Alternating Current (AC) must then be extracted by transforming - or filtering - the alternating signals into direct signals - Direct Current (DC) - that contain only the voice or sound that originated at the transmitter.
In 1890, Edouard Branly invented the first widely used detector for transforming the oscillating radio signal at the Receiver; the Branly Coherer. The Branly Coherer was based on Braun's 1874 experiments regarding the electro-magnetic properties of certain metal to mineral contacts. Branly's device used loose metal filings to provide resistance to the oscillating electrical current (AC) passing between two terminals to produce a Direct Current (DC) signal that could be ‘heard’. In 1897, Guglielmo Marconi, used his version of the Branly Coherer in the Receiver used to accomplish the first wireless communication across the English Channel. The Branly Coherer method of detection was replaced in 1907 by the Crystal Detector.
The Crystal Radio gets its name from the Crystal Detector. The Crystal Detector is a ‘Cat’s Whisker’ wire that is dragged and pressed against a Galena Crystal until a radio station or static is found. The Galena Crystal extracts the audio signal from the AC carrier wave by only allowing the current to pass through in one direction. This rectifies the AC radio wave into Direct Current (DC). The DC current is then converted into sound by the Earphone. Finding and maintaining the desired signal for any length of time was difficult and the Cat’s Whisker Diode was eventually replaced by the more modern Germanium Diode.
A Crystal Radio is a very simple AM band receiver popular in the early days of radio. The receiver gets all of its power from radio waves picked up by a long wire Antenna and does not need batteries or household current. The simplicity of the Crystal Radio design made it an ideal vehicle for the explosion of worldwide wireless communication in the early 20th Century.
People in isolated locations and without access to any power source, could scrounge the parts from local sources and assemble a rudimentary receiver capable of capturing radio signals from miles away. To rural area farmers and ranchers, the homemade Crystal Radio became a prized source of information about markets, weather and news. In urban areas, for the first time in history, large groups of people were instantly aware of events taking place in their home town and around the world. As late as World War II, soldiers in the field were using their razor blades to construct what became known as the ‘Foxhole Crystal Radio’ to listen to radio broadcasts for information and entertainment.