. The Bell System technical journal . Fig. 10—Cross-section of the barrier type transmitter used in modern handset instruments. There were reasons for supposing that the heating of the contact isa necessary factor in microphonic action. This point of view wassupported by Preece, who wrote in 1893, Indeed there are manyphenomena such as hissing and humming that are clearly due towhat is known as the Trevelyan efifect, that is, the motion set up byexpansion and contraction of bodies which are subjected to variation intemperature. This at least tends to favor the heat hypothesis asdoes also the f
. The Bell System technical journal . Fig. 10—Cross-section of the barrier type transmitter used in modern handset instruments. There were reasons for supposing that the heating of the contact isa necessary factor in microphonic action. This point of view wassupported by Preece, who wrote in 1893, Indeed there are manyphenomena such as hissing and humming that are clearly due towhat is known as the Trevelyan efifect, that is, the motion set up byexpansion and contraction of bodies which are subjected to variation intemperature. This at least tends to favor the heat hypothesis asdoes also the fact that with continuous use some transmitters becomeessentially warm. 172 BELL SYSTEM TECHNICAL JOURNAL Another view was that microphonic action arises from change inresistivity of the solid carbon resulting from strain. This view washeld by Edison who doubtless believed it because of the success ofhis microphone which was designed with the object of applyingpressure variation to a solid carbon block. It failed of general. Fig. 11—Carbon granules made from anthracite coal (X 15). acceptance because the effect of pressure on resistance, as shown byexperiment, seemed definitely to be too small. It was generally con-sidered that the Edison instrument was in fact a loose contactalthough Edison himself did not realize it. Others of the early inventors considered the contact area to be theessential element—that is to say, the extent of surface or the numberof molecules involved in intimate contact. As Professor SylvanusThompson expressed it in 1883, An extremely minute motion ofapproach or recession may suffice to alter very greatly the number ofmolecules in contact. . Just as in a system of electric lamps inparallel arc the resistance of the system increases when the number oflamps is diminished and diminishes when the number of lamps con-necting the parallel mains is increased, so it is with the molecules atthe two surfaces of contact. I THE CARBON MICROPHONE 173 Recent Theo
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