. The thermionic vacuum tube and its applications . olts. Fig. 24represents two experimental curves plotted on the logarithmicscale and obtained in such a way that in the one case (curve 2)the voltage drop in the filament was effective and in the other(curve I) it was eliminated. To eliminate the effect of thevoltage drop in the filament we can, as has been done in takingthese curves, resort to a scheme used by von Baeyer ^ in 1909,which consists in connecting the ends of the filament and the1 0. VON Baeyer, Phys. Zeits., Vol. 10, p. 168, 1909. PHYSICS OF THE THERMIONIC VALVE 69 plate through
. The thermionic vacuum tube and its applications . olts. Fig. 24represents two experimental curves plotted on the logarithmicscale and obtained in such a way that in the one case (curve 2)the voltage drop in the filament was effective and in the other(curve I) it was eliminated. To eliminate the effect of thevoltage drop in the filament we can, as has been done in takingthese curves, resort to a scheme used by von Baeyer ^ in 1909,which consists in connecting the ends of the filament and the1 0. VON Baeyer, Phys. Zeits., Vol. 10, p. 168, 1909. PHYSICS OF THE THERMIONIC VALVE 69 plate through a commutator which is so arranged that the filamentcurrent and the plate voltage are applied alternatively for shortintervals of time, the plate voltage being applied only while thefilament voltage is cut off. If the alternations are frequentenough, the filament docs not get a chance to cool off markedlyduring the time that the filament current is cut off. In this waythe plate current is measured only while there is no voltage dropin the 4 5 6 7 8 9 10 20 30 40 50 60 70 80 901 Anode Volts Fig. 24. The dotted line in Fig. 24 is drawn to have a slope equal to |.For the higher voltages curve 1 is probably close to the theoreticalline, but at the lower voltages it bends to the left on account of theinitial velocities of the emitted electrons. Curve 2 plainly showsthe effect of the voltage drop in the filament; it deviates every-where to the right of the theoretical line. Hence, the effect ofthe voltage drop in the filament is opposite to the effect caused bythe initial velocities. While the latter causes the logarithmic 70 THERMIONIC VACUUM TUBE plot of the characteristic to deviate to the left from the f-line, thevoltage drop in the filament causes not only a deviation to the rightbut also a lateral shift of the whole curve to the right. Thesetwo effects sometmies contribute to give a better f logarithmicline than is to be expected from theoretical considerations only,b
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