. The thermionic vacuum tube and its applications . Ond VoltbFig. 74 This equation was determined empirically and is subject tocertain limitations. In the first place, it does not apply to thehorizontal part of the characteristic which gives the saturationcurrent, but only to that part which obtains when the filament ishot enough to emit more electrons than are needed for the currentconvection through the tube. This is the condition under whichthe amplifier operates, because here the plate current can be THE THERMIONIC AMPLIFIER 153 varied by varying any of the applied voltages. Another condit
. The thermionic vacuum tube and its applications . Ond VoltbFig. 74 This equation was determined empirically and is subject tocertain limitations. In the first place, it does not apply to thehorizontal part of the characteristic which gives the saturationcurrent, but only to that part which obtains when the filament ishot enough to emit more electrons than are needed for the currentconvection through the tube. This is the condition under whichthe amplifier operates, because here the plate current can be THE THERMIONIC AMPLIFIER 153 varied by varying any of the applied voltages. Another conditionfor equation (4) is that the grid should not become sufficientlypositive to distort the characteristic. Under these conditions Ihave generally found this equation to hold sufficiently well, to afirst approximation at least, and have been using it in connectionwith work on the amplifier tube. The above equation does, how-ever, not hold sufficiently accurately for purposes of radio detection,since this is determined by second order 15 ZO 25 Grid Volts Fig. 75. Latour ^ has derived some equations for the relay effect of audion tubes. He starts from the general functional expressionsfor the plate and grid currents: If^F{Ep, Eg) and Ig=f{Eg, Ep).In the expansion of these equations he neglects all quantities ofthe second and higher order, thus assuming that the current andvoltage variations are very small, or that the characteristic islinear over the operating range. 1 M. Latour, Electrician, December, 1916. 154 THERMIONIC VACUUM TUBE Yallaiiri ^ also assumes a linear characteristic by exjoressingthe equation for the plate current in the form: Ip = aEg-\-hEp-\-c. It will l)e shown later that it is important to distinguish betweenthe characteristic of the tube itself and that of the tube and exter-nal circuit combined. The latter can by taking special precau-tions be made practically linear. The characteristic of the tubeitself can, however, not be regarded as hnear o
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