. American telephone practice . to exactly agree with the form ob-tained from theoretical considerations on the basis that the changein resistance is due to area of surface contact alone. As to the third explanation, it may be said that the very fact thatthe increase of current is needed to cause the rise of temperatureseems to preclude the supposition that the rise of temperatureshould cause the diminution of resistance with its corresponding * Microphonic Telephonic Action, by Professor R. A. Fessenden, American Electrician,May, 1897. CARBON TRANSMITTER. 55 increase of current in time to do
. American telephone practice . to exactly agree with the form ob-tained from theoretical considerations on the basis that the changein resistance is due to area of surface contact alone. As to the third explanation, it may be said that the very fact thatthe increase of current is needed to cause the rise of temperatureseems to preclude the supposition that the rise of temperatureshould cause the diminution of resistance with its corresponding * Microphonic Telephonic Action, by Professor R. A. Fessenden, American Electrician,May, 1897. CARBON TRANSMITTER. 55 increase of current in time to do any good. The heating effects incarbon are comparatively slow, and it would seem that the changesin temperature would lag slightly behind the changes in currentproducing them, in such a manner as to be detrimental to telephonetransmission. It is certainly most fortunate that in one substance—carbon—should be found all the qualifications which make it particularlydesirable for microphonic work. It produces the change in re-. FIG. BLAKE TRANSMITTER. sistance with changes in surface contact, all things considered,better than any other known substance, possesses the desirableproperty of lowering its resistance when heated, and is elastic, non-corrosive, non-fusible, cheap, and easily worked. The form of transmitter almost universally used in this countryup to within a few years ago, but now almost obsolete, is that de-vised by Francis Blake of Boston. This instrument is shown inFig. 40, in which B represents a metal ring or frame for holding 56 AMERICAN TELEPHOXE PRACTICE. the mechanism of the instrument. It to the cover, A\of the box A, and has two diametrically opposed lugs, B B2. Onthis ring is mounted the diaphragm, C, of rather heavy sheet-iron, supported in a rubber ring, r, stretched around its edge, andis held in place by two damping springs, D D, each bearing on asmall block of soft rubber, a, resting on the diaphragm at a pointnear its center. The o
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