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ELECTRICAL APPARATUS COMPO
with the anode, but is positive as compared with an electron, hence tends to draw hack the emitted electrons, thus opposing the anode attraction. When an electron is first emitted it is much nearer the cathode than it is to the anode; the attraction of the cathode, although weak, is close by. If the electron be emitted with feeble velocity it will return to cathode. If it be emitted at high speed its momentum will carry it far enough from the cathode to allow the anode attraction to draw it the rest of the waxacross the tube, (c) There is a cathode space charge, formed by the cloud of electrons which have been emitted at weak velocity and are in pr< f returning t-1
cathode. This cloud of negative electrons just outside the cathode surface tends to repel electrons just inside the said surface, that otherwise might be emitted; hence tends to repress emission, (d) An anode space charge resulting" from what is called secondary emission. The anode is heated by the impact of the oncoming electron-. sometimes to a white heat. Both because of this increase in temperature and as a direct result of electron bombard ment the anode produces emission. The electrons that leave it are soon dragged back by its positive charge, but in the interim they constitute a negative space charge surrounding the anode, which has some effect in repelling oncoming electrons of low velocity, driving them back to cathode instead of allowing them to complete their circuit through the vacuum.
(93) In a gas-filled tube the current at a given anode voltage is limited by the emission. I [owever, there is no cathode space charge, this being neutralized by the posi tive ions which unite with its electrons as fast as they appear, therefore electrons leaving the cathode do not return to it again. The existence near the cathode of positive ions which have not yet been neutralized tendto promote emission. A gas-tilled tube has much lower space resistance, or plate impedance, than a vacuum tube of the same structure.
(94) Tubes of the construction shown in Figs. 40 and 41 are strictly one-way conductors. Electrons can pass from cathode to anode, but not in reverse direction since