COLD CATHODE TUBES FOR A TDK) FREQUENCY SIGNALS 1373 



a chemically inert gas to a pressure ranging from 1 to 100 mm of mercury. 

 The negative electrode, called the cathode, is given a special processing 

 which permits it to emit electrons readily when bombarded with positive 

 ions of the gas. The positive electrode, the anode, serves to collect elec- 

 trons emitted by the cathode as well as those produced in the gas by 

 ionization. 



A typical voltage-current characteristic is shown in Fig. I . For discus- 

 sion purposes we may divide the curve into several current ranges. In 

 current range I a small residual current flows even at low voltages be- 

 cause of ionization resulting from cosmic rays or radio-active material 

 placed in the tube. The two curves in range I are for different residual 

 currents. At higher voltages this residual current is amplified as a result 

 of additional ions and electrons formed in the gas but it is still extremely 

 small. If the tube voltage is increased still more, the current increases 

 very rapidly until in current range II, a self-sustaining discharge is es- 

 tablished. Each electron released from the cathode gains enough energy 

 on the way to the anode so that it produces a large number of positive 

 ions, excited atoms, and photons in the gas. When these particles, arriv- 

 ing back at the cathode, on the average, release another electron, the 



10- 



10-2 



10"® io-« 10-* 



TUBE CURRENT IN AMPERES 



Fig. 1 — Volt-ampere characteristic of cold cathode diode. 



