. The thermionic vacuum tube and its applications . hese measurementswere made with a standard YT—\ tube. It will be noticedthat the best reults are obtained when the grid is kept positivewith respect to the filament. This is in accordance with the expla-nation given in Section 103, where it was shown that detectionwith a blocking condenser in the grid ciicuit depends on the 358 THERMIONIC VACUUM TUBE curvature of the grid current characteristic. The detectingcurrent increases with increase in the curvature of this char-acteristic and also becomes greater the steeper the plate cur-rent grid po
. The thermionic vacuum tube and its applications . hese measurementswere made with a standard YT—\ tube. It will be noticedthat the best reults are obtained when the grid is kept positivewith respect to the filament. This is in accordance with the expla-nation given in Section 103, where it was shown that detectionwith a blocking condenser in the grid ciicuit depends on the 358 THERMIONIC VACUUM TUBE curvature of the grid current characteristic. The detectingcurrent increases with increase in the curvature of this char-acteristic and also becomes greater the steeper the plate cur-rent grid potential characteristic is. If the relation betweenthe maximum detecting current for a constant voltage of the gridbattery be plotted as a function of the plate batteiy voltage acurve is obtained such as that obtained in Fig. 214. This curveshows a rapid increase in detecting current with increase in theplate voltage until a plate battery voltage of about 34 volts isreached. The bend in the curve at higher voltages is due to the 50x10 40 30 ?20. 0 l•^ -i^a Fig. 213. plate current characteristic becoming less steep when voltagesaturation is approached. 110. Zero Beat or Homodyne Method of Receiving ModulatedWaves. The heterodyne method of reception can be used whenreceiving continuous unmodulated waves, as is done in radiotelegraphy, but cannot be applied to the reception of modulatedwaves as in radio telephony. In this case we can make use of thesimple systems which consist in first detecting the modulatedhigh frequency waves and then amplifying the low frequencycomponent with the ampHfier tube attached to the output of the DETECTION OF CURRENTS WITH THE VACUUM TUBE 359 detector. Or the received high frequency currents can first beamplified and then detected. This latter scheme has the advan-tage of greater sensitivity because the voltage impressed on theinput of the detector which is connected to the output of theamplifier is amplified and the detecting
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