. The thermionic vacuum tube and its applications . ents are made. Theprinciple of this method can be explained with reference to The incoming high frequency oscillations are impressedon the grid in the usual way. In order to measure the small 1 H. J. VAN UER BiJL, Phys. Rev., Vol. 13, p. 311, 1919; Proc. Inst. RadioEngineers, Vol. 7, p. G03, 1919. DETECTION OF CURRENTS WITH THE VACUUM TUBE 337 detecting current in the output of the detector we use a generatorU, giving a note of the same pitch as that of the detecting current,and then attenuate the current from the generator by means
. The thermionic vacuum tube and its applications . ents are made. Theprinciple of this method can be explained with reference to The incoming high frequency oscillations are impressedon the grid in the usual way. In order to measure the small 1 H. J. VAN UER BiJL, Phys. Rev., Vol. 13, p. 311, 1919; Proc. Inst. RadioEngineers, Vol. 7, p. G03, 1919. DETECTION OF CURRENTS WITH THE VACUUM TUBE 337 detecting current in the output of the detector we use a generatorU, giving a note of the same pitch as that of the detecting current,and then attenuate the current from the generator by means of areceiver shunt S until the current id has the same value as thedetecting current deUvered by the tube. W is a switch wherebythe telephone receiver can be connected either to the output ofthe tube or to the output of the generator. The shunt and seriesresistances of the receiver shunt are adjusted until the tone heardin the receiver is of the same intensity for both positions of theswitch W. The receiver shunt has been described in Section t/Q / \/W\A/W\///>A/ WAwOwAV) Fig. 202. The shunt and series resistances are varied in definite steps by thesimple operation of turning a dial, these steps being so propor-tioned that the impedance in the output of the generator Uremams constant for all adjustments of the shunt. The currenti\ delivered by the generator into this impedance is so large thatit can easily be measured with a hot wire instalment A such as athermo-couple. It was shown in Section 72 that the relationbetween the current i\ and the branch current id flowing throughthe receiver is ii td (28) where a is the constant of the receiver shunt and d expresses thecurrent attenuation produced by the shunt in terms of length of 338 THERMIONIC VACUUM TUBE the cable or line having an attenuation constant equal to a perunit length. For the standard cable of reference commonlyused in telephony a = per mile, d being expressed in miles(see Section 72). Expressing the
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