Can You Hear Me Now?
January 17, 1882: Carbon Microphone for Telephone
Thomas Edison created more tech history when he was granted a patent for the carbon microphone for the telephone. The carbon was in between two cups and attached to the mouthpiece. The diaphragm would change pressure, causing the carbon to be pushed upon. That would send electric current to the other end – which would transpose it back to audio.
carbon microphone n: (Engineering / Electrical Engineering) a microphone in which a diaphragm, vibrated by sound waves, applies a varying pressure to a container packed with carbon granules, altering the resistance of the carbon. A current flowing through the carbon is thus modulated at the frequency of the sound waves.
The device using several pieces of the semi-conductor instead of one was early tried by Mr. Edison. He found in general that the loudness of the sound was increased by thus multiplying the number of contact-surfaces, but also that the articulation was impaired. Instruments of this nature have since become known as microphones, though it is not probable that faint sounds were ever augmented through their agency.
Fig. 9 shows one of the first forms of the microphone, invented by Mr. Edison, April 1, 1877. Four pieces of charcoal are used, C C, etc., each supported by an upright spring, as at S and S’. The piece of charcoal nearest the diaphragm impinges upon a disk of carbon, which is fastened to the centre of the diaphragm. The primary wires of an induction-coil are attached to the diaphragm and the spring S’. The circuit is then completed through the semi-conductors.
Before the proliferation of vacuum tube amplifiers in the 1920s, carbon microphones were the only practical means of obtaining high-level audio signals, and were widely used in telephone systems. Their low cost, inherently high output and “peaked” frequency response characteristic were well suited for this application, and their use in new telephone installations continued up to the 1980s, long after they had been replaced by other types of microphones in other applications. (In most Western copper-wire telephone networks, old fashioned carbon-microphone based telephones can still be used without modification.)
After protracted patent litigation, in 1892 a federal court ruled that Edison—and not Emile Berliner—was the inventor of the carbon microphone. Berliner’s legacy also lives on in his trademark, a picture of a dog listening to his master’s voice being played from a gramophone. The dog’s name was Nipper.
Carbon microphones were widely used in early AM radio broadcasting systems (usually modified telephone microphones), but their limited frequency response, as well as a fairly high noise level, led to their abandonment for that use by the late 1920s. They continued to be widely used for low-end public address, and military and amateur radio applications for some decades afterward.
Apart from legacy telephone installations in Third World countries, carbon microphones are still used today in certain niche applications in the developed world. An example is the Shure 104c, which is still in demand because of its wide compatibility with existing equipment.