. The thermionic vacuum tube and its applications . Fig. 54. table on p. 77 it is seen that if the filament is of tungsten and isoperated at a temperature a little under 2500° K. the thermionicefficiency is 10 per watt. Since the necessary saturation cur-rent /i is 300 the power necessary for heating the filament ofthe valve under consideration is 30 watts. The power availablein the form of unidirectional current in the load can be obtained. Fig. . from equation (15). Putting /2 = 0, the heating equivalent of thecurrent in the load resistance is g -^ and the available power in th
. The thermionic vacuum tube and its applications . Fig. 54. table on p. 77 it is seen that if the filament is of tungsten and isoperated at a temperature a little under 2500° K. the thermionicefficiency is 10 per watt. Since the necessary saturation cur-rent /i is 300 the power necessary for heating the filament ofthe valve under consideration is 30 watts. The power availablein the form of unidirectional current in the load can be obtained. Fig. . from equation (15). Putting /2 = 0, the heating equivalent of thecurrent in the load resistance is g -^ and the available power in the form of unidirectional current is g^ -^. Assuming that the current-voltage characteristic of the circuit is, in view of thehigh load resistance of about 100,000 ohms, practically linear, 132 THERMIONIC VACUUM TUBE we can put g = .707. This makes the available power about\KW, which is quite large compared with the power necessary toheat the filament. It is, in fact, so nmch larger that it is advan-tageous to quadruple the filament heating power in order to doublethe output power with the arrangement shown in Fig. 55. Now, what is ordinarily observed in practice is not the heatingcurrent ?o which must be measured with an a-c. meter, but thetrue mean of the unidirectional current i measured with anordinary d-c. ammeter. The output power can then be readilyobtained from i-pr, where / is the form factor (equation 17),assuming that the valve does
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