The
carbon microphoneA microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
, also known as a
carbon button microphone (or sometimes just a
button microphone) or a
carbon transmitter, is a sound-to-electrical signal
transducerA transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
consisting of two metal plates separated by granules of carbon. One plate faces outward and acts as a
diaphragmIn the field of acoustics, a diaphragm is a transducer intended to faithfully inter-convert mechanical motion and sound. It is commonly constructed of a thin membrane or sheet of various materials. The varying air pressure of the sound waves imparts vibrations onto the diaphragm which can then be...
. When sound waves strike this plate, the pressure on the granules changes, which in turn changes the
electrical resistanceThe electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
between the plates. (Higher pressure lowers the resistance as the granules are pushed closer together.) A direct current is passed from one plate to the other, and the changing resistance results in a changing current, which can be passed through a telephone system, or used in other ways in electronics systems to change the sound into an electrical signal.
Before the proliferation of
vacuum tubeIn electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
amplifiers in the 1920s, carbon microphones were the only practical means of obtaining strong 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for this application, and their use in new telephone installations continued up to the 1980s, long after they had been replaced by other types of microphones in other applications.
Carbon microphones were widely used in early AM radio broadcasting systems (usually modified telephone microphones), but their limited
frequency responseFrequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input...
, 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.
History
The invention of the carbon microphone (then called a "transmitter") was claimed both by Thomas Alva Edison in March 1877 and separately by
Emile BerlinerEmile Berliner or Emil Berliner was a German-born American inventor. He is best known for developing the disc record gramophone...
who filed related patent applications in June 1877 and August 1879. The two sides fought a long legal battle over the patent rights. Ultimately a federal court awarded Edison full rights to the invention of the carbon microphone, saying "Edison preceded Berliner in the transmission of speech...The use of carbon in a transmitter is, beyond controversy, the invention of Edison" and the Berliner patent was ruled invalid. British courts also ruled in favor of Edison over Berliner. Having settled the Dowd suit (after Peter A. Dowd, agent of
Western UnionThe Western Union Company is a financial services and communications company based in the United States. Its North American headquarters is in Englewood, Colorado. Up until 2006, Western Union was the best-known U.S...
) out of court in 1881, Western Union left the telephone business, and sold Edison's patent rights and related assets to the
Bell companyThe Bell System was the American Bell Telephone Company and then, subsequently, AT&T led system which provided telephone services to much of the United States and Canada from 1877 to 1984, at various times as a monopoly. In 1984, the company was broken up into separate companies, by a U.S...
in exchange for 20% of telephone rental receipts. Subsequently Bell telephones used the Bell receiver and the Edison transmitter. Later, carbon granules were used between carbon buttons. Carbon microphones were widely used in telephones in the United States from 1890 until the 1980s.
Carbon microphones used as amplifiers
One of the surprising attributes of carbon microphones is that they can actually be used as
amplifierGenerally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
s. This capability was used in early telephone repeaters, making long distance phone calls possible in the era before vacuum tube amplifiers. In these repeaters, a magnetic telephone receiver (an electrical-to-mechanical
transducerA transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
) was mechanically coupled to a carbon microphone. Because a carbon microphone works by varying a current passed through it, instead of generating a signal voltage as with most other microphone types, this arrangement could be used to boost weak signals and send them down the line. These amplifiers were mostly abandoned with the development of
vacuum tubeIn electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
s, which offered higher
gainIn electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...
and better
sound qualitySound quality is the quality of the audio output from various electronic devices. Sound quality can be defined as the degree of accuracy with which a device records or emits the original sound waves...
. Even after vacuum tubes were in common use, carbon amplifiers continued to be used during the 1930s in portable audio equipment such as hearing aids. The Western Electric 65A carbon amplifier was 1.2" in diameter and 0.4" high and weighed less than 1.4 ounces. Such carbon amplifiers did not require the heavy bulky batteries and power supplies used by vacuum tube amplifiers. By the 1950s, carbon amplifiers for hearing aids had been replaced by miniature vacuum tubes (only to be shortly replaced by transistors). However, carbon amplifiers are still being produced and sold.
One illustration of the amplification provided by carbon microphones was the oscillation caused by feedback, that resulted in an audible squeal from the old "
candlestick telephoneThe candlestick telephone is a style of telephone that was common from the late 1890s to the 1930s.-History:Also known as a desk stand, upright, or a stick phone, it came with or without a rotary dial, and with a ringer box...
" if its earphone was placed near the carbon microphone.
Early radio
Early
AMAmplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
radio transmitters relied on carbon microphones for voice modulation of the radio signal. In the first long-distance
audioSound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
transmissions by
Reginald FessendenReginald Aubrey Fessenden , a naturalized American citizen born in Canada, was an inventor who performed pioneering experiments in radio, including early—and possibly the first—radio transmissions of voice and music...
in 1906, a
continuous waveA continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
from an
Alexanderson alternatorAn Alexanderson alternator is a rotating machine invented by Ernst Alexanderson in 1904 for the generation of high frequency alternating current up to 100 kHz, for use as a radio transmitter...
was fed directly to the transmitting antenna through a water-cooled carbon microphone. Later systems using
vacuum tubeIn electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
oscillators often used the output from a carbon microphone to modulate the
grid biasGrid bias is a DC voltage applied to electron tubes with three electrodes or more, such as triodes. The control grid of these devices is used to control the electron flow from the heated cathode to the positively charged anode...
of the oscillator or output tube to achieve modulation.
Current usage
Apart from legacy telephone installations in
Third WorldThe term Third World arose during the Cold War to define countries that remained non-aligned with either capitalism and NATO , or communism and the Soviet Union...
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.
The principal advantage carbon microphones have over other microphone types is that they can produce high-level audio signals from very low DC voltages, without needing any form of additional amplification or batteries. Most all-electronic telephones need at least 3 volts DC to work, whereas carbon transmitter telephones will continue to work down to a fraction of a volt. Electronic telephones also suffer from the so-called "cliff effect", whereby they abruptly stop working when the line voltage falls below the critical level, while a carbon microphone on the same line would simply have reduced output.
Carbon microphones are also widely used in safety-critical applications such as mining and chemical manufacture, where higher line voltages cannot be used, due to the risk of sparking and consequent explosions. Carbon-based telephone systems are also resistant to damage from high-voltage transients such as those produced by lightning strikes and
electromagnetic pulseAn electromagnetic pulse is a burst of electromagnetic radiation. The abrupt pulse of electromagnetic radiation usually results from certain types of high energy explosions, especially a nuclear explosion, or from a suddenly fluctuating magnetic field...
s of the type generated by nuclear explosions, and so are still maintained as backup communication systems in critical military installations.
External links
- Edison's invention of the carbon (graphite) microphone by Frank Dyer
- T A Edison, Speaking Telegraph (graphite microphone), filed April 1877, issued May 1892
- T A Edison, Improvement in Speaking Telephones (compressed lamp black button insulated from diaphragm), filed March 1878, issued April, 1878
- T A Edison, Carbon Telephone (carbon granules microphone), filed Nov 1878, issued Dec 1879
- E. Berliner, Improvement in Electrical Contact Telephones (carbon diaphragm with carbon contact pin), filed August 1879, issued December 16, 1879
- A C White, Telephone (solid back carbon microphone), filed March 24, 1892, issued November 1, 1892 (Bell engineer)