STIMULATORS 



circuit now has the appearance of Figure 38.15, and clearly if R^ + R^ is 

 much smaller than R^ + ^45 ^3 + -^4 are 'swamped' — i.e. have little effect, 

 and 'i?-balance' can be effected ; the artefact can be simplified to type (d) in 

 Figure 38.13. Dickinson' shows a stimulator possessing this facility. Strictly 

 speaking it should be possible to achieve C 'balance' with a differential 

 variable capacitor, and the time constant balance with an additional resistor, 



a"- '.a. 



Figure 38.15 



in the arrangement shown in Figure 38.16, but the author has never heard 

 of anyone attempting it. Probably it would be much too difficult to set up, 

 and in any case Wagner earth-type balancing is a risky procedure. The 

 reason is that the balancing current which flows to earth at point E re-enters 

 the preparation at the earthing connection D. It is this earth current which 

 is liable to cause the anomalous stimulation mentioned earlier. We therefore 

 fall back on method 2. 



Maximizing the bridge arm impedances — In practical terms this means that 

 the site of stimulation should be as far as possible from the recording site 



constant 

 balance 



Figure 38.16 



(maximum R^ and R^ and the stimulator and recording wiring should be 

 separated as widely as possible (minimum Q and C^. So much is common- 

 sense, and this analysis would not be justified if these were the only con- 

 clusions. What is less obvious, and equally important, is that the stimulator 

 output circuit should have no conductive connection to earth at all (other 

 than the inevitable path through the preparation — maximum R^ and R^ and 

 the minimum possible stray capacitance to earth (minimum C3 and CJ. 



612 



