. Electronic apparatus for biological research . Figure the grid bias and raising the anode potential until ionization occurs, we can of course fix the anode potential at some value greater than the ionization potential and initiate conduction by raising the grid potential from very negative to [anode potential]/[control ratio]. When this is done (bearing in mind that it subsequently loses control) the grid is said to be being used as a 'trigger' and—extending the analogy—when ionization occurs the thyratron is said to 'fire'. The only way to stop the discharge again is to reduce V below
. Electronic apparatus for biological research . Figure the grid bias and raising the anode potential until ionization occurs, we can of course fix the anode potential at some value greater than the ionization potential and initiate conduction by raising the grid potential from very negative to [anode potential]/[control ratio]. When this is done (bearing in mind that it subsequently loses control) the grid is said to be being used as a 'trigger' and—extending the analogy—when ionization occurs the thyratron is said to 'fire'. The only way to stop the discharge again is to reduce V below the ionization potential. Precautionary measures to be taken with thyratrons are similar to those for soft hot-cathode diodes. The cathode must be allowed to warm up before the anode potential is applied, and the discharge current must be held within the makers' upper limit, for fear of destroying the cathode. Thyratron relaxation oscillator—One difficulty with the difference diode relaxation oscillator is that the amphtude of the three outputs cannot readily be controlled electronically. Output (1) is fixed by the voltage-diff'erential of the tube, and outputs (2) and (3) by the relative sizes of R^ and R^, {Figure ). The thyratron relaxation oscillator can be controlled by a potential on the thyratron grid {Figure ). The action is similar to the difference diode oscillator, but whilst the capacitance always discharges to the ionization potential, the voltage to which it charges can be made greater by making the grid more negative. Unfortunately the frequency of oscillation is also affected and has to be corrected by an appropriate alteration in R. In designing these circuits it must be remembered that R^ + Rb must be chosen so that the peak thyratron current is within the safe limit when the grid is as much negative as it is ever likely to be, for it is then that the capaci- tance charges to the highest voltages and the discharge currents are greatest, that is M
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