STIMULATORS 



of affairs by two potential dividers, R1R2 and R^R^, as shown in Figure 

 38.11b. We now put in the stray capacitances between A and B and the 

 recording electrode, calling them Q and Co, and between A and B and 

 earth, calling them C3 and C4, giving us Figure 38.12a. Finally, let the 

 stimulator now become a constant-voltage pulse generator, of output E, 

 and let the amplifier be replaced by its input impedance, Z, assumed to be 

 relatively large {Figure 38.12b). 



The problem of minimization of stimulus artefact is now seen to be that 

 of minimizing the out-of-balance current in a bridge circuit, but before 



©^ 



S '^a^is 



tTi 





(b) 



Figure 38.12 



attending to this it is worth remarking the nature of the unbalance current. 

 It has three components : a transient of magnitude 



C, 



C. 



[Ci + Cg C3 -|- C4J 

 a final value which it reaches if the pulse goes on long enough of magnitude 



^ ii?l + i?2 i?3 + RJ 



and a phase between them which is the difference of two exponentials cor- 

 responding to the time constants 



(^'+^^>(^J 



and 



(C3 + C4) 



' RzR, ^ 



The observed artefact on the cathode ray tube face may have any of the 

 forms in Figure 38.13. — but only a, b, or c are at all likely. It is also worth 

 noting at this point that the bridge cannot possibly be balanced if one side 

 of the stimulator output is earthed, i.e. if R^ or 7?4 is zero. This puts all the 



610 



