. The thermionic vacuum tube and its applications . atisfies condition (3)and has a characteristic such as that shown in Fig. 35 (6). Itconsists of a filament which can be heated to incandescence and aplate, both placed in a highly evacuated bulb. In 1905 Fleming ^recognized the use of the rectifying properties of this device forthe indication of high frequency oscillations, and used it as a 1 J. A. Fleming, Proc. Roy. Soc, Jan., 1905, p. 476; U. S. Pat., 803, 684. 112 THERMIONIC VACUUM TUBE radio detector. The circuit in which Marconi used this deviceas a radio detector is shown in Fiji;. 39.
. The thermionic vacuum tube and its applications . atisfies condition (3)and has a characteristic such as that shown in Fig. 35 (6). Itconsists of a filament which can be heated to incandescence and aplate, both placed in a highly evacuated bulb. In 1905 Fleming ^recognized the use of the rectifying properties of this device forthe indication of high frequency oscillations, and used it as a 1 J. A. Fleming, Proc. Roy. Soc, Jan., 1905, p. 476; U. S. Pat., 803, 684. 112 THERMIONIC VACUUM TUBE radio detector. The circuit in which Marconi used this deviceas a radio detector is shown in Fiji;. 39. 45. Valve Detector with Auxiliary Anode Battery. By ourpresent standard of measurement the two-electrode tube is avery inefficient detector. It can be, and has been used moreefficiently by operating on a chosen point of the current-voltagecharacteristic, thus making it fall in the class of rectifiers given bycondition (4) instead of that represented by condition (3). Thiscan be done by inserting a local battery in the circuit as shownin Fig. The operation of the device when used this way can be under-stood from the following: By the insertion of the battery E thereis established in the circuit FPE a constant direct current whichhas a finite value even when no oscillations are impressed from theantenna. The current through the device can therefore be repre-sented by a function of the form ?/+z=/(i?+f sin/^O, (5) where E is the local source of direct voltage, / the direct currentdue to E and i the superposed a-c. due to e. This oan be expandedinto a power series: j{E+e sin vt) -=f{E) +f(E)e sin pt ^r{E) e^ sin- j)t +/^(^) e sin pt iSee Lee de Forest, Proc. , Vol. 25, p. 719, 1906, and J. , Proc. Roy. Inst., Great Britain, June, 1909, p. 677. RECTIFICATION OF CURRENTS BY THERMIONIC VALVE 113that is l + i=fm+I{Ef^^ . . H(E)e sin pt -f(Ef^^^+ ... (6) If now this be integrated over a complete period the sineand cosine terms vanish. Of the remainder the term f(
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