BIOLOGICAL AMPLIFIERS 



(9) Microelectrode resistance estimation. Connect the microelectrode to 

 the cathode follower input and dip it into a vessel of earthed Ringer's fluid. 

 Apply 1 millivolt to the cathode follower input terminal from the calibrator 

 (switched to 'direct') and observe the deflection produced on the cathode ray 

 trace. Now operate the calibrator switch to insert resistance in series with 

 the calibrator output until the deflection produced on the trace is halved. 

 The amount of series resistance inserted is then equal to the microelectrode 

 resistance (Figure 39.14). 



1 mV step 



Calibration 

 resistor 



HT + 



\jmxi 



Figure 39. 14 



Other calibrators — The calibrating device just described is not very accurate, 

 depending as it does on the freshness of the cell. Moreover it is useful to 

 have other calibrating potentials beside 1 mV available. A more sophisti- 

 cated calibrator is shown in Figure 39.15. This gives up to 1 V in 1 mV steps, 

 but the scheme may be extended to give smaller increments. The pre-set 

 potentiometer is used to set the circulating current to exactly 1 mA as 

 indicated on the meter. The 10 O. potentiometer is of course continuously 

 variable but has a scale marked 0-10. The assumption is that operating 

 this potentiometer does not affect the circulating current. Clearly with a 

 total circuit resistance of about 1,500 Q. this is substantially true; the error 

 is less than 1 per cent and the meter cannot be read to better than this. 

 Purists may open the circuit at a point such as 'X' and insert another 10 Q. 

 variable resistor, ganged to the first and arranged so that as the first increases 

 the new one decreases. 



An even more accurate calibrator is used by the Rockefeller unit of the 

 Physiological Laboratory in Cambridge in which the output can be checked 

 against a Weston's standard cell {Figure 39.16). Since the e.m.f. of a Weston 

 ceH is 1-0186 V, if the circulating current be adjusted until there is no micro- 

 ammeter deflection on pressing the check button, then it must be equal to 

 100 fxA. 



Carrier amplifiers 



We have seen that these comprise a d.c.-a.c. converter or modulator, an 

 a.c. coupled amplifier and a phase sensitive rectifier. The modulator may be 

 mechanical, in which case it comprises a vibrating relay or capacitor, or it 



628 



