Journal . he cathode,rapidly to cause the destruction of the latter. The presence ofgases in a vacuum valve also causes the formation of negativeelectrons so that on test a greater current flow is shown but itsbehaviour is very erratic. For this reason a hard valve ispreferred in practice in spite of the difficulty of removing theoccluded gases from the metal parts and the glass bulb. The third element or grid is interposed to control thecurrent flow through the valve by variation of its potential,its presence introduces great possibilities, as slight variations ofvoltage impressed upon it can
Journal . he cathode,rapidly to cause the destruction of the latter. The presence ofgases in a vacuum valve also causes the formation of negativeelectrons so that on test a greater current flow is shown but itsbehaviour is very erratic. For this reason a hard valve ispreferred in practice in spite of the difficulty of removing theoccluded gases from the metal parts and the glass bulb. The third element or grid is interposed to control thecurrent flow through the valve by variation of its potential,its presence introduces great possibilities, as slight variations ofvoltage impressed upon it can either check the current flow Id 98 IV rv » ?> Id I- 7 < -J •• •• s X ••• st f\ . y a i. 6v - O-flJ r° 3 3 s *c 4v?ITICAl = 0-6 . V * 4- 30 40 • r € 9 •a M.« ;.qc c ) O0 •4- MC3 MC II IS Fig. 3 -O + 3 BELOW Voltage of Grid. 2.—Flow of Clrrent CONTROLLED by Grid (see p. 49) 25 40 SOME MODERN ENGINEERING PRACTICE IN AMERICA WIRELESS VALVES. Fig. 3. I Z 3 4 Filament Flow on Tests (see p. 51). entirely, when acting in opposition, or increase the flow manyfold when a positive potential is impressed upon it. (Fig. 2.) The sizes and disposition of all the elements in a valve areworked out with the greatest degree of accuracy, the satisfactoryoperation in service depends upon accurate manufacture, sothat the problem is quantity production of exactly similarapparatus within very narrow limits. Further the fact mustbe borne in mind that the majority of the faults liable to creepinto such manufacture can only be detected when the articleis practically completed and the electrical tests are this time, if a fault has developed in a jig or a process, pro-bably a great number of pieces are on the way having the samefault. SOME MODERN ENGINEERING PRACTICE IN AMERICA. 41 t40
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