COUPLING THE PREPARATION TO THE STIMULATOR 



electronic stimulators we have discussed so far — as they stand — straight out 

 of court. 



To reduce the artefact one can either : 



(1) attempt to balance the bridge by juggling with the ratios of the im- 

 pedances of the arms. 



y 



Neither R nor 

 C balanced 



R balanced, C 

 unbalanced 



C balanced, R 

 unbalanced 



R's and C's balanced 

 but time constants 

 unbalanced 



Everything balanced 



Figure 38.13 



(2) make the impedances of all the arms as high as possible. Applying 

 Thevenin's theorem to the bridge, we see that looking into it from the ampli- 

 fier we have its open-circuit e.m.f , determined by its degree of off-balance, 

 in series with its internal impedance, determined by the impedance of its 



Figure 38.14 



arms. If we cannot correct the unbalance current by method 1, we can at 

 least reduce it by keeping the arm impedances as high as possible. 



Balancing the bridge — We can partially balance the bridge by what might 

 be described as 'brute force' — using a Wagner earth. This is done by connect- 

 ing between the stimulator terminals a potentiometer of much lower resis- 

 tance than i?3 + i?4, and earthing the slider {Figure 38.14). The bridge 



611 



