1378 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



In the higher current range IV and V the impedance can be represented 

 by a positive resistance in series with an inductive reactance. The re- 

 sistance increases slowly with frequency as indicated by the elhptical 

 current-voltage loci at 200 and 2,000 cps. 



The impedance of a cold cathode tube also varies with anode-to- 

 cathode spacing. This may be studied by means of the same movable- 

 anode parallel-plane tube used to obtain the data of Fig. 3. We again 

 operate the tube in the current range of the normal glow (IV) and, for 

 illustrative purposes, choose a measuring frequency of 1,000 cycles per 

 second. The results are shown in Fig. 5. The resistive and reactive com- 

 ponents of tube impedance are independent of anode spacing throughout 

 the Faraday dark space and well into the obstructed discharge region. 

 At large distances where the electron space charge sheath begins to build 

 up in front of the anode, the impedance increases rapidly with distance. 



Some useful conclusions about the design of transmission tubes can be 

 drawn from the data of Fig. 5. We can consider the total tube impedance 

 as being made up of the sum of the impedances introduced by the various 

 regions of the discharge. Since large variations in the length of the 

 Faraday dark space do not affect tube impedance, it is concluded that 

 this region has negligible impedance. This means that, so long as the 

 anode-to-cathode distance is short enough to avoid a space charge sheath 

 at the anode, we can concentrate our attention on the cathode fall region. 

 The following detailed discussions of impedance are consequently re- 

 stricted to the cathode portions of the glow discharge. 



Fig. 4 — Static and dynamic volt-ampere characteristics 



