OTHER ELECTRODES 



The advantages of direct heating are its simplicity and the rapidity with 

 which the cathode reaches its operating temperature. The disadvantages are: 

 (1) that the heater supply must necessarily be at cathode potential, so that a 

 number of valves having their cathodes at different potentials will need a 

 separate heater battery or transformer winding each, and (2) there is a 

 potential gradient along the cathode by virtue of the voltage dropped across 

 it which means that it is a moot point what the 'cathode potential' is. 



INDIRECTLY HEATED CATHODE 



Here the cathode is a nickel tube coated with highly emissive material and 

 filled with a refractory insulating material such as alumina ; threaded through 

 it is the heating element, a hairpin-shaped piece of tungsten wire (Figure 8.2). 



Heater 



Refractory 

 filling 



BaO 



Symbol 



fTJ^"''' 



Cathode 

 lead-in wire 



U. 



Heater 



lead -in wires 



Figure 8.2 



Such a cathode has a large thermal capacity and may be heated by a.c. or 

 d.c. The cathode and the heater are insulated from each other, so that a 

 number of valves with different cathode potentials may be fed from a common 

 heater supply; furthermore the cathode surface is equipotential. On the 

 whole the indirectly heated cathode is the more versatile device, and valves 

 possessing them are much more common in electrobiological work. 



OTHER ELECTRODES 



Around the cathode is the anode, usually approximately cyhndrical and made 

 of nickel. Between them, according to the type of valve, there may be up to 

 three 'grids'; these are concentric helices of nickel wire. There is also 

 another object in the valve called a 'getter', but this is concerned with the 

 manufacturing process and not with the operation of the valve. 



In biological ampHfiers valves are often 'under-run' — that is, the cathode 

 temperature is arranged to be lower than that intended by the manufacturers. 

 As we shall see later this is dictated by necessity; as a general rule it should 

 be avoided, and care should be taken that the heater voltage is correct and 



133 

 10 ^-^-^ 



