. The Bell System technical journal . ating space current in a tube must, for reasons which will 36 BELL SYSTEM TECHNICAL JOURNAL appear later, be appreciably less than the saturation current. The dif-ference between the saturation current and the maximum operatingspace current varies with the duty to which the tube is assigned. Inthe case of high voltage rectifiers, the space current may at certainpoints in the cycle reach the saturation value, while in a tube which isused as an amplifier it is often desirable, in order to avoid distortion, tohave the total emission two to three times as grea
. The Bell System technical journal . ating space current in a tube must, for reasons which will 36 BELL SYSTEM TECHNICAL JOURNAL appear later, be appreciably less than the saturation current. The dif-ference between the saturation current and the maximum operatingspace current varies with the duty to which the tube is assigned. Inthe case of high voltage rectifiers, the space current may at certainpoints in the cycle reach the saturation value, while in a tube which isused as an amplifier it is often desirable, in order to avoid distortion, tohave the total emission two to three times as great as the maximumworking space current. 3. Space Current-Voltage Characteristic. Experiment shows thatin a vacuum tube containing an emitting electrode and a convenientlyplaced anode, the space current Ip, varies with the temperature ofthe emitter and the anode potential Ep, as in Fig. 4. The three curvesshown are for three temperatures such that Ti<T2<T3. It will beobserved that between points 0 and A the three curves coincide;. Anode PotentialFig. 4 between 0 and B the curves for the two higher temperatures saturation values of the filament emission at the various temper-atures are shown by Is. For values of TP ranging from zero to somewhat less than Is, therelation between IP and EP may be expressed with a fair degree ofaccuracy by Ip = kEJ in which the exponent v does not differ greatlyfrom 3/2. This relation, therefore, is frequently known as the 3/2power law. It has been deduced theoretically by Child 7 and Lang-muir 8 and has been studied lately in greater detail by Frysanalysis takes account of the initial velocities of emission of electrons, 7 Phys. Rev., Vol. 32, p. 498, 1911. *Phys. Rev. (2), Vol. 2, p. 450, 1913. •T. C. Fry, Phys. Rev., Vol. 17, p. 441, 1921. THERMIONIC VACUUM TUBES 37 and, as he shows, the effect of the space charge is to create a region ofnegative potential immediately around the emitter. Let Fig. 5represent the value of
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