STIMULATORS 



the primary of a 1 to 4 step-up transformer. The cold-cathode trigger 

 tetrode has its HT arranged so that the main gap just does not break down, 

 and the 0-01 capacitor C charges to 240 V. Each time a pulse arrives from the 

 oscillator via the trigger electrode, the tube fires and the capacitor discharges 

 via the 10k potentiometer across which the output is developed. The 33k 

 resistor is to limit the discharge current. On the completion of discharge the 

 capacitor rapidly re-charges to 240 V via the 22k resistor. The waveform of 

 the output pulse is now independent of the frequency settings. 



Output 



250n 



Ik 



10k 



100k 



IM 



•-\ 



Figure 38.5 



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 38.5). 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 



