598 BELL SYSTEM TECHNICAL JOURNAL 



tube stages. With an even number of stages the polarity will be 

 reversed from that given by an odd number. This is because an 

 increase in negative potential on the grid of a vacuum tube causes a 

 decrease in the space current and hence a decrease in the negative 

 potential applied to the grid of the next tube. 



In the case of the grid type of lamp with the individual external 

 electrodes, the impedance to which energy must be supplied differs 

 materially from that presented by the rectangular electrode lamp 

 already described. For low voltages the impedance between any 

 electrode and the central helix is effectively a capacitance of the order 

 of 6 m.m.f. When, however, the voltage gradient in the interior of 

 the tube becomes sufficient to break down the gas and cause a dis- 

 charge to take place, the capacitance is increased to about 15 m.m.f. 

 In fact, the tube may be looked upon as consisting of two capacitances 

 connected in series. When the applied potential is sufficient to break 

 down the gas and cause a glow discharge, that capacitance corre- 

 sponding to the portion of the path inside the tube is effectively 

 shunted by an ohmic resistance. The minimum discharge potential 

 has been found to be independent of frequency over a wide range, but 

 the current between electrodes is inversely proportional to the fre- 

 quency because of the presence of the capacitance between the electrode 

 and the glowing gas. Now, the brightness of the discharge is a 

 function of the current sustaining it so that it becomes desirable to 

 use high frequencies in order to get sufficient light without going to 

 prohibitively high potentials. It is also desirable to operate at such 

 a portion of the frequency scale that the percentage difference between 

 the limits of the range shall be small, thus avoiding signal distortion 

 due to the effect referred to above. There is, however, a definite 

 upper limit to the frequency beyond which it would be impossible to 

 operate because of the stray capacitances in the cable connecting the 

 grid to the distributor. It has been found feasible to operate at a 

 frequency of the order of a half million cycles. 



The circuit problem, therefore, involves the production of a high 

 frequency wave which varies in amplitude in accordance with the 

 amplitude of the received picture signal. The solution has been con- 

 veniently obtained by using a radio broadcast transmitter the voice 

 frequency circuits of which have been so modified that the extended 

 range of frequencies required might be handled with minimum dis- 

 tortion. 



The envelope of the 500-kilocycle wave modulated by the picture 

 signal, as shown in Fig. 28, is proportional to the signal amplitude plus 

 a direct current biasing component of such magnitude that when the 



