SOFT VALVES 



'Soft' valves are those in which the electrodes are sealed in a glass envelope 

 containing either an inert gas or mercury vapour at a reduced pressure. The hst 

 is nearly but not quite complete if we tabulate them in the following manner: 



Hot Cathode 



Cold Cathode 



Two-electrode 



Three-electrode 



Electrical conduction in metals is possible because of the presence within 

 the matrix of the material of a large number of free, unbound electrons. 

 Due to the thermal agitation of the molecules of which the metal is composed 

 electrons are continually being shot out of the metal altogether (thermionic 

 emission); usually they merely fall back into the metal again. The average 

 rate at which electrons are emitted is not unexpectedly a function of the 

 temperature of the metal and of a constant for the metal itself. This is of the 

 form / = AT^e~^'^ where Tis the absolute temperature, a relationship known 

 variously as Richardson's or Dushman's equation. 



COLD CATHODE DIODE 



If two electrodes are connected to a source of voltage and immersed in an 

 inert gas, then an electron casually emitted at the cathode experiences an 

 attraction towards the anode and will not necessarily fall back at once into 

 the cathode again: instead it may hit a gas molecule. If the electron hits 

 the gas molecule sufficiently hard it will ionize it, producing another electron 

 and a positive ion. This molecular disruption will occur when the potential 

 difference across the device exceeds a certain critical value called the ioniza- 

 tion potential. Positive ions so created set off towards the cathode and elec- 

 trons towards the anode. A minute current flows through the valve. If the 

 potential difference is further increased a point is reached at which the heavy 

 positive ion on hitting the cathode knocks out further electrons and the chain 

 reaction is established at which the cathode and anode become joined by a 

 glowing, conducting column. The potential difference at which this occurs is 

 the 'striking voltage'. 



A chain reaction is an unstable state of affairs and requires 'moderating'; 

 if a cold cathode diode or any other soft valve be connected to a constant- 

 voltage supply above the striking potential the current rises at once to an 

 indefinitely high value, at which the device is destroyed. To remedy this a 



118 



