1376 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



122 

 120 

 118 

 116 

 114 

 112 



O 



Z 110 



z 



H 108 



^ 106 



104 



102 



100 



0.04 0.06 0.08 0.10 0.12 0.14 



ANODE CATHODE SPACING IN INCHES 



0.16 



0.18 



Fig. 3 — Sustaining voltage as a function of anode-cathode spacing. 



of magnetic material which can be moved by a magnet external to the 

 tube. Fig. 3 shows a typical set of data obtained in this manner with an 

 operating current in the normal glow range of current. At very close 

 spacings of the order of the negative glow distance, the voltage is higher 

 than normal because some of the electrons released from the cathode are 

 able to strike the anode before dissipating their energy in producing ions 

 and excited particles in the gas. This is referred to as an "obstructed 

 discharge".^ The voltage must, therefore, be higher because electrons 

 which do produce excitation or ionization must be given extra energy in 

 order to maintain the current. As the anode is moved away from the 

 cathode through the Faraday dark space, the tube sustaining voltage 

 stays at a fairly constant minimum value. This is possible because at 

 close spacings more than a sufficient number of electrons and ions are 

 present in the Faraday dark space to carry the current. As the anode is 

 moved away the Faraday dark space is lengthened. Ions and electrons 

 needed to carry the current diffuse from the cathode region. At suffi- 

 ciently large distances, however, the ionization density decreases so that 

 not enough ions are present to cancel space charge near the anode. A 

 space charge sheath builds up in the anode region and the sustaining 

 voltage rises with increasing distance. 



When the voltage has increased by about 10 to 18 volts depending upon 

 the gas filling, it begins to level off with increase in distance. At about 

 this point, an anode glow may appear in front of the anode. This is due 



