GLASS MICROCAPILLARY ELECTRODES 



with small cells, such as internuncial neurones, and possibly electrodes with 

 a sharp taper near the tip may be of help. However, Frank and Fuortes^^ 

 found it necessary to use electrodes with resistances 50-100 MQ. or more. 

 To reduce the time constant, electronic correction can be used, which can 

 give an effective input capacitance less than 1 pF. 



When a voltage step is applied to a fluid bath with the microelectrode 

 immersed, as in the recording position, the voltage waveform shows an 

 initial sudden step followed by an exponential rising portion up to the final 

 level (Figure 35.11). The initial step is caused by the displacement of charge 



Figure 35.11 Equivalent circuit of a microelectrode dipped in a fluid bath 

 through which it receives a pulse 



in the wall capacitance in series with the voltage source. The height of the 

 step is determined by the ratio of the electrode wall capacitance (Q) to the 

 input capacitance of the cathode follower (Qn), and is proportional to 

 CeliQn + O- The slowly rising portion represents the process of charging 

 both capacitors through the electrode and input resistances in parallel, with 

 a time constant therefore of 



where 7?in = input resistance of amplifier and R^ = electrode resistance. 

 If the input resistance is much larger than that of the electrode then this 

 simplifies to: 



(Cin + CXRe) 



The time constant can be determined from this. 



Reduction and correction of distortion — It has been shown that the output 

 differs from the input voltage in expression 3 by an amount 



dv 



All three factors can be used to improve the response of the system. 



A correction based on the rate of change of voltage has been applied by 

 Woodbury^^, at the output of the amplifier. The voltage from this is differen- 

 tiated by means of a resistance-capacitance network of short-time constant, 

 whose output is then added to the original in a mixer stage. 



A method of reducing the effective capacitance has been described by 

 Solms, Nastuk and Alexander^" and similar methods have been developed 

 by McNichol and Wagner^^ and Haapanen and Ottoson^^. Positive feedback 

 is applied from the output of the cathode follower back to the input so that 



554 



