244 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



the curves are identical with those of vacuum tubes. With further 

 increase of the applied voltage, the current rises with increasing 

 slope. If the voltage is raised too high, a self-sustained glow dis- 

 charge will set in which may damage the cathode surface. Operation 

 of the tubes at voltages below this critical value, and the insertion of 



40 60 



Anode voltage, volts 



80 



100 



Fig. (5-1)10. Voltage-current characteristic of a gas- 

 filled photoelectric cell {from RCA Tube Manual, Tube 

 1P-41; by permission). 



a resistor to limit the current in case of an accidental overvoltage are 

 recommended. 



The output current is not strictly a linear function of the light flux; 

 at higher levels of illumination in particular, the curve presenting 

 output current versus incident light flux rises with increased slope. 



Since the current in a gas tube is carried by electrons as well as by 

 positive ions, it is reasonable to expect a time lag in the response, 

 which is not observed in a vacuum tube. Figure (5-1)11 shows the 

 current output for a silver-cesium oxide-cesium phototube con- 

 taining argon at a pressure of 0.15 mm Hg, operated at different 

 voltages and irradiated with light pulses. For light signals modu- 

 lated sinusoidally at different frequencies, the output drops off at 

 higher frequencies, Fig. (5-1)12. For some applications the drop of 

 the frequency characteristic may be compensated for by subsequent 

 networks. 



Dark currents in commercial gas tubes are, in general, of the order 

 of 10 -8 to 10 -7 amp. Gas phototubes are available with the same 

 spectral characteristics in the visible and infrared as vacuum photo- 

 tubes. Their application is primarily in the field of relay operation 



