GB165793A - Improvements in or relating to systems of long distance telephony - Google Patents

Abstract

165,793. Naamlooze Vennootschap de Nederlandsche Thermo - Telephoon Maatschappij. July 5, 1920, [Convention date]. Telephone systems.-In a system of long-distance telephony, a thermophone is combined with an amplifier to form a receiver sensitive also to the higher frequencies, thus compensating for the unequal damping produced in the line. The thermophone a is connected through the amplifier c and a transformer d to the line wires f. To prevent interference between the transmitting microphone k and the adjacent receiver, the primary of the transformer d is divided into two parts e, e<1>, to a point between which the microphone transformer g is connected, a compensating artificial conductor consisting of resistances w and condensers o being connected as shown. By suitable adjustments of the intervalve transformers in the amplifier c, a constant amplifying ratio may be obtained over a definite range of frequencies. The resistance and impedance of the transformer d may be made negligible compared with those of the mircophone transformer, thus supplying the whole of the current energy to the line. The same local battery may be used for microphone, thermophone, and amplifier, or central battery arrangements may be employed. Microphones having direct-current power of from 5 to 10 watts may be used, for example, the Egner-Holmstr÷m microphone.