. The thermionic vacuum tube and its applications . to the pos-itive end the condition for maximum detecting current is Ey = has also been verified experimentally. The conditionEy = 0 when the grid is connected to the positive end of the filamentdoes, of course, not mean that the space current is zero becausesince Ey is positive when reckoned from all points on the filamentother than the extreme positive end. When using the tube in the simple circuit shown in Fig. 195it is necessary to make sure that electrons do not flow to the is usually secured by putting in the negative gr
. The thermionic vacuum tube and its applications . to the pos-itive end the condition for maximum detecting current is Ey = has also been verified experimentally. The conditionEy = 0 when the grid is connected to the positive end of the filamentdoes, of course, not mean that the space current is zero becausesince Ey is positive when reckoned from all points on the filamentother than the extreme positive end. When using the tube in the simple circuit shown in Fig. 195it is necessary to make sure that electrons do not flow to the is usually secured by putting in the negative grid battery Eg. DETECTION OF CURRENTS WITH THE VACUUM TUBE 331 In practice, especially when receiving weak signals, it is usuallynot necessary to insert this battery because the potential varia-tions impressed on the grid seldom exceed a small fraction of a volt,and, under these conditions, the current flowing to the grid isusually negligibly small. There is, however, a factor which mustbe considered, namely the contact potential difference between. 40 fcO Plate Voltage Fig. 197. the filament and the system constituting grid and plate. Thequantity e in the characteristic equation gives a measure of thiseffect. If the filament, grid and plate are of the same material ewill usually be practically zero, but if the filament is, for example,of a different material € may be either positive or negative, but itseldom exceeds the value of about 1 volt. If it is positive it meansthat the grid is intrinsically positive with respect to the filamentand, therefore, to secure best operation it is necessary to insert 332 THERMIONIC VACUUM TUBE a grid batteiy to maintain the resultant potential of the grianegative with respect to the negative end of the filament. Inthe case of tubes containing oxide coated filaments, e is usually-negative. In such a case, therefore, the grid battery can be dis-pensed with altogether. The quantity e will differ from zerowhenever the electron affinity of the fil
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