I was speaking with my Grandfather the other day about early crystal radios. This is how he would listen to the radio in California during the 1930s:
"In the 1930s, I would buy the crystal set which contained:
1) A fixed-germanium diode mounted in a lead base
2) A whisker wire
3) A set of ear phones
4) A copper antenna which I would hang in my bedroom
I would probe the crystal with the whisker wire until I found its rectifying property of turning an alternating radio signal into a direct signal - usually a faint voice. The federal government had designed many powerful clear channel radio stations. The only station that I could receive was the Los Angeles radio station KFI. I wasn't a ham radio operator but I had my fun."
Wikipedia on diodes:
"In 1874 German scientist Karl Ferdinand Braun discovered the "unilateral conduction" of crystals. Braun patented the crystal rectifier in 1899. Copper oxide and selenium rectifiers were developed for power applications in the 1930s.
Indian scientist Jagadish Chandra Bose was the first to use a crystal for detecting radio waves in 1894. The crystal detector was developed into a practical device for wireless radio reception by Greenleaf Whittier Pickard, who invented a silicon crystal detector in 1903 and received a patent for it on November 20, 1906. Other experimenters tried a variety of other substances, of which the most widely used was the mineral galena (lead sulfide). Other substances offered slightly better performance, but galena was most widely used because it had the advantage of being cheap and easy to obtain. The crystal detector in these early radio sets consisted of an adjustable wire point-contact (the so-called "cat's whisker") which could be manually moved over the face of the crystal in order to obtain optimum signal. This troublesome device was quickly superseded by thermionic diodes, but the crystal detector later returned to dominant use with the advent of inexpensive fixed-germanium diodes in the 1950s."
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