COLD CATHODE GAS TUBES FOR TELEPHONE SWITCHIXG SYSTEMS 763 



field results in an increase in the ionization coefficient rj, and, in some 

 cases,'^ a larger effective value of the secondary emission coefficient 7. 

 This is because electrons released by the secondary emission processes 

 may strike neutral gas atoms and be reflected back to the cathode. A 

 higher gradient increases the probability of escape of such an electron. 

 Thus the multiplication factor M of (9) can ecjual unity at a lower total 

 applied voltage. 



Practical tubes filled with neon or argon gas have sustaining voltages 

 near 100 volts when pure molybdenum or tungsten cathodes are used. 

 Cathodes coated with barium and strontium oxide may sustain at 60 

 volts. However, since this lower sustain is accompanied by a lower 

 breakdown voltage, the difference between them is not increased. Also, 

 since the coated cathode surface is more variable between tubes and with 

 tube operation, the switching voltage gain may be reduced with such 

 cathodes. 



The gas pressure and cathode geometry determine the length of the 

 flat portion DE of Fig. 2. Over this current range, the area covered by 

 the glow discharge increases with current until at E the cathode is 

 completely covered. Increasing the cathode area or gas pressure increases 

 the total current required for coverage. At still larger currents, the sus- 

 taining voltage increases rapidly as indicated by the solid curve EF. 

 Broken curve EF' applies to a special cathode geometry called a hollow 

 cathode.* Such a cathode may be formed by the interior of a cylinder or 

 by placing two plane cathodes close together so that the negative glow 

 regions overlap. Under this condition electrons, ions, and excited atoms 

 generated near one cathode can aid in current flow from the other 

 cathode. Dotted curve EF" applies to a particular form of hollow 

 cathode^ in which cathode shape and gas pressure have been selected 

 to give a negative slope in the high current region. This negative slope 

 represents a negative resistance and permits audio-frequency signals 

 to be transmitted through the tube without loss. 



Anode effects have not been discussed. In general, the anode shape 

 and location do not affect the sustaining voltage or the ability of the dis- 

 charge to transmit audio frequency unless the anode-cathode spacing is 

 too large. The basic requirement is that the anode should be large 

 enough to intercept enough electrons to carry whatever current is re- 

 (luired by the external circuit. Even a small anode placed near the 

 cathode space-charge region can meet this requirement. Thus the sus- 

 taining voltage of a tube designed to have a breakdown voltage near 

 the minimum of Fig. '.] or Fig. 4 will not in general be sensitive to the 

 anode size or shape. 



