Show lob wo 10 lot i aw Z aa W THIRD ELEMENt IN lN THE VACUUM TUBE grid added to fleming valve b by y dr lee deforest was a big improvement any device which will pass electricity in one direction and will wholly or partially obstruct the flow la in the opposite direction Is termed a rectifier because when connected in the path of an alternating current it will suppress one halt half of each cycle and therefore ahe he circuit will be traversed by pulsating direel current A rectifier also Is said to possess unidirectional conductivity meaning of course that it will ft conduct electricity in one dl III u of tin tilt grid circuit IJ K L the llie battery lit in tile the grid circuit Is culled called tb the A C battery As a start let ui suppose suppo that lint C altory battery at tory vita lol riKe e is 7 PI ro the opera biot rif or ii the three el tule tube would then in be exactly like abat of it 11 two to E alec lec lulu just us its there werf no grid like a two electrode tube felien lien the cl CD C D l Is brought to ence by the A battery a of electrons will be given off which will vill he be drawe over to tile plate E iante late H Is at a potential with rf respect hect ct to the nt by tile B battery now it if the grid la Is made positive with respect to the filament flIn trient it Is possible to t accelerate the now flow of the electron stream froin the filament to tile the plate II if the grid Is 14 made negative alth rt respect to the filament filanie ut the flow ot of the electron stream from the filament to the plate will be retarded or in other words by making the grid positive or negative with respect to the filament it Is possible to increase or counteract the space charge charee the third elee electrode frode or grid thus offers a means of controlling the current in the jill only its ability to rectify currents of extremely high frequency determines ter mines its appt leation in III radio due to its ability to rectify high frequency alternating currents the two element filament and plate vacuum tube can be used in a radio receiver ai a detector fig VI Is a simple radio receiving circuit employing this type tipe of two element vacuum tube in place of a crystal detector dete e tor dr D r J A fleming of london eng land was the first to use a two boele ele ment tube of the type just described ns as a medium of rectifying rectify ing high fre quency radio currents fleming called his hl product a val ae ve because it would let current flow in n one direction but not in the other direction the fleming valve as a forerunner of the acu bacu plate circuit without changing the plate potential or the filament temper the characteristic curve of a three electrode vacuum tube la is shown in fig als VIII mil this diagram shows the icia tion or of grid potential to plate current assuming that the filament temperature and plate voltage remain constant it can lie be seen from the curve that lint by applying a negative potential of value 13 to the grid the plate current cm can he reduced to zero the negative charge on the grid gild will have tile the effect of a negative potential B H with respect to tile the filani ent making the negative chang e so strong aro around und the filament diament that the he electrons cannot leave it on the other land hand if 1 a positive potential of value F be applied fa C 4 Z 0 0 are um tube of today smirked a very ory important step in the progress op of tile the radio art the fleming valve mihe however lit in its original forin vas not much better than aher forms form 3 of rectifiers then alieu in use and owing to the greater gi enter lu and en ease e of of df the he latter did not come into gener a use its as a elector detector dr lee reforest Ue De Forest Foyest an american greatly improved tile fleming valve ne by adding adill nj a third electrode electr oile callot the grid alisch alch served sen oil the function of a control dement aind thus mule made it possible to tn the hig nal currents to ti control diore po powerful erful local currenti the three electrode il anle tulie tube of I ik bovet ti Is the tube used so ho extensively bodily tin the third element fildi 11 Il li 19 called a grid bild froin which tilt chri e elov vacuum neuit nu uit tubo tube derives der ls its iti it i noine was placed 1 1 fielure el t between the filament und and tile the plate lit in the path of the electrons the grid gild Is a perforated plate or mesh of line fine wire ie through the openings of which the electrons must pass in their journey froin tile the filament to the tha plate fig VII la Is a sketch of the circuits of a three electrode electr oile vacuum tube aud Is identically the same ame as the sketch in fig ill for a 0 two crement vacuum tube with the ail ad to the grid with respect to tu tile III ala ment tile the or saturation ciu rent will flow lit in tilt plate cir circuit cult at applying a greater positive pot elal than ali in F to the grid gild with etli ri to tile the filament will not lot camise tin nit e in the f plute plate current ohp electrons given off me are being to the lie pent and grid when the grit in ie maintained 1 tive with respect ct to the filament a small current will all flow in the grid circuit because of its ilg being heans it will ill attract the electrons wid B ul we me a I 1 charge ghen eh en up to it ay y 11 vai iu |