. The thermionic vacuum tube and its applications . t chapter. The cathode C can be raised to any desired temperature. Letthe anode P be raised to a potential Vi, while the cathode remainsgrounded. As long as the temperature of C is so low that practi-cally no electrons are emitted, the potential gradient between the ^ J. J. Thomson, Conduction of Electricity through Gases, 2d Ed., p. 223. 2 C. D. Child, Phys. Rev. Vol. 32, p. 498, 1911. 31. Langmuir, Phys. Rev., (2), Vol. 2, p. 450, 1913. * ScHOTTKY, Jahrb. d. Radioaktivitat u. Elektronik, Vol. 12, p. 147, 1915, 56 THERMIONIC VACUUM TUBE plat
. The thermionic vacuum tube and its applications . t chapter. The cathode C can be raised to any desired temperature. Letthe anode P be raised to a potential Vi, while the cathode remainsgrounded. As long as the temperature of C is so low that practi-cally no electrons are emitted, the potential gradient between the ^ J. J. Thomson, Conduction of Electricity through Gases, 2d Ed., p. 223. 2 C. D. Child, Phys. Rev. Vol. 32, p. 498, 1911. 31. Langmuir, Phys. Rev., (2), Vol. 2, p. 450, 1913. * ScHOTTKY, Jahrb. d. Radioaktivitat u. Elektronik, Vol. 12, p. 147, 1915, 56 THERMIONIC VACUUM TUBE plates Is practically constant so that the potential distribution is alinear function of A^, the distance from the plate C, and can berepresented by a straight hne OP. But this is no longer the casewhen C emits electrons. These electrons are pulled over to P bythe apphed electric field and their presence in the space betweenC and P modifies the potential distribution. In Section 9 it wasshown that if the plates C and P are infinitelj large so that the. Distance from Coi+hodeFig. 19. lines of force are straight the potential T at any point distant xfrom C is given b} cPV ^==-*^ ^1) where p is the volume density of the charge. Now p is itself afunction of x. Thus (PVd7= = ^-/« (2) The relation between the potential T and the distance x cannot beobtained unless the form of the function x is known. We knowthat the density of electrons near C is greater than near P. Itcan therefore be seen in a general way that because of the presence PHYSICS OF THE THERMIONIC VALVE 57 of the electrons the potential distribution curve will be some-what of the nature shown by OAP (Fig. 19). The potential gradi-ent at the cathode is still positive, but if the temperature of thecathode be now raised until the potential difference appliedbetween C and P is not high enough to draw all the emittedelectrons away from Ci and if it is assumed that the electronsare emitted from C with zero velocity, the potentia
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