File:Snitt genom en Western electrics kolkornsmikrofon med dubbelsidigt verkande membran, Nordisk familjebok.png

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From https://runeberg.org/nfcr/0059.html

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English: Section diagram of Western Electric double-button carbon microphone, from Nordisk familjebok. Developed around 1921, it was widely used for radio broadcasting and recording in the 1920s and 30s. It consists of a duralumin diaphragm (A) with "button" cells containing carbon granules (C) attached to each side, in the center. Electric current from a battery flowed through the carbon, so when sound waves vibrated the diaphragm, it compressed and decompressed the carbon, changing its conductivity and thus the current. The two carbon buttons were connected to either side of a center-tapped audio transformer, which cancelled the even-order harmonic distortion from the carbon. The stretched diaphragm had a high resonant frequency, and an air chamber provided damping, flattening the frequency response.

This image was first published in the 1st (1876–1899), 2nd (1904–1926) or 3rd (1923–1937) edition of Nordisk familjebok. The copyrights for that book have expired and this image is in the public domain, because images had no named authors and the book was published more than 70 years ago.

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