. American telephone practice . the soundsdie out (as if the speaker was talking and walking away at the sametime) until a point is arrived at where there is complete silence. 20 AMERICAN TELEPHONE PRACTICE. Only one radical improvement now remains to be recorded. In1881 Henry Hunnings devised a transmitter wherein the variableresistance medium consisted of a mass of finely divided carbongranules held between two conducting plates. His transmitteris shown in Fig. 20. Between the metal diaphragm, A, and aparallel conducting plate, B, both of which are securely mountedin a case formed by the blo
. American telephone practice . the soundsdie out (as if the speaker was talking and walking away at the sametime) until a point is arrived at where there is complete silence. 20 AMERICAN TELEPHONE PRACTICE. Only one radical improvement now remains to be recorded. In1881 Henry Hunnings devised a transmitter wherein the variableresistance medium consisted of a mass of finely divided carbongranules held between two conducting plates. His transmitteris shown in Fig. 20. Between the metal diaphragm, A, and aparallel conducting plate, B, both of which are securely mountedin a case formed by the block, D, and a mouthpiece, F, is a chamberfilled with fine granules of carbon, C. The diaphragm, A, and theplate, B, form the terminals of the transmitter, and the current fromthe battery must therefore flow through the mass of granular car-bon, C. When the diaphragm is caused to vibrate by sound waves,it is brought into more or less intimate contact with the carbongranules and causes a varying pressure between them. The resist-. FIG. 20.—HUNNINGS GRANULAR CARBON TRANSMITTER. ance offered by them to the current is thus varied, and the desiredundulations in the current produced. This transmitter, instead ofhaving one or a few points of variable contact, is seen to have amultitude of them. It can carry a larger current without heating,and at the same time produce greater changes in its resistance, thanthe forms previously devised, and no ordinary sound can cause atotal break between the electrodes. These and other advantageshave caused this type in one form or another to largely displace allothers. At first the practice was to put the transmitter, together with thereceiver and battery, directly in circuit with the line wire. With THE BATTERY TRANSMITTER. 21 this arrangement the changes produced in the resistance by thetransmitter were small in comparison with the total resistance ofthe circuit, especially in the case of a long line, and the changes incurrent were therefore small.
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