COLD CATHODE GAS TUBES FOR TELEPHONE SWITCHING SYSTEMS 707 



age at the operating current of 10 nia is U!) ± 2 volts. Thus the .switching 

 gain from (5) is (180 101)/20 or o.9. In practice, switching is often done 

 without allowing the full 100 milliamperes operating current to flow. 

 Under these conditions, the sustaining voltage may be 10 or 15 \-olts 

 higher, with a conseciuent reduction in switching ^'oltage gain. 



Short breakdown times were desired for this tube. It was not desirable 

 to use enough radium to obtain the needed initial ionization, since it is 

 expected that large numbers of these tubes will be concentrated in a 

 relatively small space. Also, the molybdenum cathode does not emit 

 photoelectrons unless short wavelength ultraviolet illumination is used. 

 The solution chosen was to use the barium getter flash as an auxiliary 

 photocathode. An electrical contact is made to the getter deposit and 

 this is connected through a high resistance to the main cathode. Visible 

 light or long wave ultraviolet light is readily transmitted through the 

 bulb and produces photoelectric current in the auxiliary gap. This cur- 

 rent is amplified b}^ the gas, but remains a non-self-sustaining discharge. 

 Currents of 10"^" amperes are readily available with a few foot-candles of 

 illumination. This current is too small to afi'ect the breakdown voltage 

 of the main gap, but produces enough residual ionization to allow break- 

 down times of the order of 100 microseconds to be obtained with a few 

 volts overvoltage. The high resistance connection to the main cathode 

 may be of the order of 20 to 50 megohms. It protects the photocathode 

 from deterioration which might result from high currents when the main 

 gap is conducting. 



Recovery of breakdown voltage following conduction is rapid. Meas- 

 urements indicate that the breakdown \'oltage is within the limits of 

 190 ± 10 volts in less than 500 microseconds. The relatively high gas 

 pressure and close spacings speed up the deionization process. 



The tube described has not been designed for large scale manufacture 

 although several hundred models have been made and tested to establish 

 the feasibility of the design. 



SUMMARY 



Some useful switching properties of gas diodes can be described by 

 defining the switchiug-voltage gain. This gain is shown to be equal to 

 the difference between the breakdown and the sustaining voltage divided 

 by the variation in the breakdown voltage. The gain is reduced if faster 

 switching times are required. 



The switching-voltage gain is discussed in terms of the physical 

 processes in a gas discharge. It is shown that a high gain can be obtained 

 by using an inefficient anode operating at the minimum of the curve 



