GLASS MICROCAPILLARY ELECTRODES 



until it is charged, so that the output voltage will follow an exponential 

 course, The current, /, at any instant, /, flowing through a resistance, R, 

 causes a voltage drop iR 



and iR = F, - v„ .... (1) 



Fluid, 

 bath 



Tissue- 



Amplifier 







(a) 



R 

 rAAAAAt 



^'© 



X 



(b) 



Figure 35.10 (a) Representation of a recording arrangement; 

 {b) equivalent circuit of (a) 



(2) 



Substituting for / from expression 1 , expression 2 becomes 



dv 



(3) 



The voltage Vg at any time t is less than V^ by an amount equal to a constant 

 of the circuit (RC) times the rate of change of voltage at that time (see also 

 Figure 35.12 and Chapter 3), When the voltage reaches a constant level 

 dvjdt = and F^ = v^. 

 The solution of expression 3 is 



See Figure 3.9 



When t = CR seconds 



-^ t 

 v,= V,(\-e ^«) 



-^<(-3 



(4) 



i.e. at t = CR the voltage has risen to 63 per cent of its final value. The 

 product RC, the time constant, can be determined from the response curve 

 on applying voltage step and finding the time taken to reach 63 per cent of 

 the final steady value. 



552 



