STIMULATORS 



assembled in tin boxes measuring only 7 x 4 x 2J in. complete with 

 battery, and have proved satisfactory for medical students' practical work. 



For research work something rather more elaborate is required. In 

 repetitive operation the repetition frequency, in addition to the shock ampli- 

 tude, ought to be controllable over a wide range. A well-tried way of achiev- 

 ing this is the simple 'neon stimulator' {Figure 38.3) which is the relaxation 



I— ^"^-A^AA^ 

 J- 2M 



5M 



I 

 240V I 

 I 

 I 



0-01 

 0-03 

 0-1 

 0-3 

 1 



Output 



Shock 

 ampli'tude 



Figure 38.3 The symbol — x • • • • x — means that the component between 

 the crosses is switchable to any of the values shown 



oscillator described in Chapter 7. Coarse control of shock frequency is 

 obtained by switching in various capacitors between, say, 0-01 and 1 /^F, 

 and fine control by having part of the charging resistance variable. For the 

 'neon' lamp a difference diode characteristic is desirable, but in point of fact 

 almost any kind of cold-cathode diode will do. Old voltage stabilizer tubes 

 whose regulating properties have long since departed are quite suitable; 

 sometimes more output is obtainable if they are connected up the wrong way 

 round. 



+240 



Figure 38.4 



A drawback of the simple neon stimulator is that the time constant, and 

 to some extent the amplitude, of the output depends on the charging capaci- 

 tor in use; that is, frequency and shock waveform are not completely 

 independent. If this is undesirable recourse may be had to two cold-cathode 

 valves, one to generate the repetitive frequency and one to fashion the shock. 

 The circuit of Figure 38.4, devised by the author, gives shocks of from zero 

 to 10 V amplitude at any repetition frequency between 1 and 100 per second. 

 The circuit asssociated with the SI 30 tube is a straightforward relaxation 

 oscillator, like Figure 38.3, except that the load is not a potentiometer but 



603 



