. The thermionic vacuum tube and its applications . a difficult matter becauseit involves the measurement of veiy small alternating values under practical conditions range from about 10~^ampere down to 10~^ ampere and sometimes less. This makes it1E. H. Armstrong, El. World, Vol. 64, p. 1149, 1914. 336 THERMIONIC VACUUM TUBE entirely impossible to use hot wire instruments. The telephonereceiver is a very sensitive device for indicating small alternating;currents, but does not directly give a measure of the value of thecurrents in the receiver. The audibihty method, which will be
. The thermionic vacuum tube and its applications . a difficult matter becauseit involves the measurement of veiy small alternating values under practical conditions range from about 10~^ampere down to 10~^ ampere and sometimes less. This makes it1E. H. Armstrong, El. World, Vol. 64, p. 1149, 1914. 336 THERMIONIC VACUUM TUBE entirely impossible to use hot wire instruments. The telephonereceiver is a very sensitive device for indicating small alternating;currents, but does not directly give a measure of the value of thecurrents in the receiver. The audibihty method, which will bediscussed later on, has been suggested to measure detecting cur-rents with a telephone receiver. It consists in shunting the tele-phone receiver with a variable resistance and adjusting thisresistance until the current in the telephone receiver is just largeenough to make it possible to discriminate between the dots anddashes of the incoming signals. The ratio of the total currentin the receiver and shunt resistance, that is, the detecting current. Plafe Vol+age Fig. 201. to the current in the receiver alone, measures what is known as the audibility. This method is not very reliable, and its accuracydepends to a large extent on the conditions under which themeasurements are made (see Section 108). The following method requires only that two notes of the samepitch be adjusted to equal intensities.^ It is, comparativelyspeaking, very accurate, and does not depend nearly so much onthe conditions under which the measurements 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 th
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