HARD VALVES 



that some 'high tension' is always applied to the valve. If the HT is absent 

 or if the heater voltage is too lovv' then the cathode is liable to be 'poisoned' 

 and its emission fail. If the HT is too high, i.e. exceeds the upper limit set 

 by the manufacturers, the power dissipated in the valve will be excessive and 

 the anode will overheat ; when this happens there is a risk of the liberation 

 of absorbed gas and consequent cathode destruction by ionic bombardment. 

 If the heater voltage is excessive the cathode will overheat and the valve will 

 fail from premature evaporation of the barium and strontium oxides de- 

 posited on it. 



DIODE 



If a hard diode current be measured as a function of the potential applied to 

 the anode {Figure 8.3), the resulting relationship is of the form shown in Figure 

 8.4. The portions of the curve AB, BC, and CD represent three distinct 

 regimes : 



AB. Here the anode is negative with respect to the cathode, and electrons 



A B 



C^ D 



Figure 8.3 



Figure 8.4 



Figure 8.5 



emitted by the cathode are turned back by the repulsive field due to the anode: 

 no current flows. 



BC. Anode moderately positive with respect to the cathode. Here the 

 average rate at which electrons break clear of the cathode depends upon the 

 resultant of the repulsive effect due to the negative space charge of electrons 

 already emitted, and the attractive effect of the anode. The current is said to 

 be 'space charge limited' and is proportional to V^l^. This is called Child's 

 law, or merely the 'three halves power law'. At C electrons are being removed 

 from the cathode region to the anode as fast as they can be emitted, and the 

 space charge disappears. If V be further increased there can be no cor- 

 responding increase in current because it cannot exceed the electron emission 

 rate given by the Richardson equation. 



CD. Between C and D the current is said to be 'temperature limited' and 

 the diode is 'saturated'. If we are working near the point C between C and D, 

 reversion to space charge limited working can be had by raising the cathode 

 temperature, for the curve then flattens out at a higher value o^ I {Figure 8.5). 

 A saturated diode is important because it is an approximation to a constant- 

 current generator, provided the anode voltage is kept high enough. Special 

 valves are made for this purpose, since ordinary valves are used under space 



134 



