. Electronic apparatus for biological research . Figure The magnitude of the output of 'neon' or cold cathode stimulators is of the order of 10 V, sufficient for excised preparations but of little use where there is much by-passing of stimulating current by surrounding structures. More powerful outputs can be had from thyratrons^, but there seems to be an increasing tendency among research workers to demand stimulating pulses of rectangular waveform; the repetition rate, duration and amplitude all to be variable. To do this it is usual to employ hard valves. An early hard-valve stimulator
. Electronic apparatus for biological research . Figure The magnitude of the output of 'neon' or cold cathode stimulators is of the order of 10 V, sufficient for excised preparations but of little use where there is much by-passing of stimulating current by surrounding structures. More powerful outputs can be had from thyratrons^, but there seems to be an increasing tendency among research workers to demand stimulating pulses of rectangular waveform; the repetition rate, duration and amplitude all to be variable. To do this it is usual to employ hard valves. An early hard-valve stimulator was described in 1944 by Ritchie^; this comprised a pair of power triodes arranged as a multi-vibrator {Figure ). The output is delivered when Kg conducts and has duration determined by the time constant C^/?!. The range of pulse durations obtainable was 25 /isec to 100 msec. The interval between pulses, when F^ conducts and Fg is cut off, is determined by R^C^ and ranged from 1 to 1/50 of a second. Notice that when the output is derived from an anode load the positive power supply terminal is earthed. A drawback of a simple stimulator such as this is that when the shock duration and shock interval are comparable, and the duration is varied, the shock frequency is varied too. On the whole it is probably more satisfactory to be able to set a definite frequency, independent of the shock duration, and this may be done by using a multi-vibrator to control a flip-flop. Two rather similar stimulators along these lines are due to Bernstein^ and to Ead^. Both of these have in addition an output stage to buffer the flip-flop from the effect of variations in the load. Bernstein uses an output valve arranged as an amplifier and derives a negative-going stimulus pulse. Ead 604
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