PRODUCTION OF TELEVISION SIGNALS 597 



Curve A expresses the relation between the grid potential of the 

 vacuum tube and its plate current. Curve B shows the relation 

 between this same plate current and the voltage across the external 

 resistance. When no current is flowing through the vacuum tube, the 

 potential of the biasing battery is insufficient to break down the neon 

 lamp and no current flows through the circuit containing the neon 

 lamp and the plate circuit resistance. As the current through the 

 vacuum tube is increased from zero, the total current flowing is that 

 through the resistance branch. When, however, the potential drop 

 across this resistance reaches such a magnitude that, together with 

 the potential of the biasing battery, it is sufficient to break down the 

 neon lamp, the latter will begin to draw current which thereafter 

 increases linearly with further increases in the voltage, Ex, across 

 the external resistance. The voltage across the neon lamp itself 

 differs from that across the resistance by the amount of the battery 

 Eb- The relation between the neon lamp current and the voltage 

 across it, as given by Curve C, may therefore be plotted directly above 

 the characteristic just discussed by displacing the vertical axis an 

 amount corresponding to Eb- This amount is shown as Eli- Here 

 again we have two separate biases controlled by a single adjustment. 

 The potential Eli is fixed by the minimum plate current which can 

 be taken from the tube without departing too seriously from the linear 

 portion of the tube characteristic. It is, therefore, an operating bias 

 of the circuit which is unaffected by any characteristic of the neon 

 lamp. The latter, however, must be operated with a bias E^ corre- 

 sponding to its efifective back e.m.f. As in the case of the grid circuit 

 bias just considered, the bias Eli actually introduced into the circuit 

 is the difference between these two independently determined biases. 



By projecting values of lamp current horizontally and plotting 

 their intersections with vertical projections through the corresponding 

 grid potentials on the vacuum tube characteristic we obtain Curve D, 

 which expresses the relation between the instantaneous value of the 

 signal and of the current in the neon lamp as derived from the charac- 

 teristics of the several elements of the circuit. Inasmuch as the 

 intensity of the illumination is proportional to the lamp current, it 

 will be seen that we have approached the desired linear correspondence 

 between the instantaneous values of the signal and of the light. 



It will be noted that care has to be exercised to insure that the 

 alternating current as impressed on the last vacuum tube is of the 

 proper polarity. If it is not, the received image will be a negative 

 instead of a positive. This may be controlled either by the con- 

 nections to any one of the transformers or by the number of vacuum 



