154 



THE SIGNS OF LIFE 



Note. Obviously this yields only approximate results, since the 

 voltage of a Leclanche is never quite 1.4 volt, and the fraction 



of the voltage taken is not - but 



Further, the internal 



R + r. 



resistance of the cell is not taken into account. The method is, 

 however, sufficiently accurate, seeing that the principal error is 

 a constant one and the variable error is negligible. 



In actual experiments it is advisable to have a standard com- 

 pensator which will give values of .01, .001, .0001 volt, independently 

 of the compensator proper. 



90 14100 



FlG. 57 A. Compensator to deliver nrihnrth, nsVijth, or 

 volt from a Leclanche cell (of 1.42 volt). 



o th of a 



The comparison between experimental deflections and the 

 standard deflection of known external voltage is not calculated 

 to give the absolute value of internal E.M.F. of active tissue. The 

 external circuit and galvanometer receive only a fraction of the 

 total internal electromotive difference, which produces current 

 partly through the internal conducting tissues, partly through the 

 external (galvanometric) arc. Moreover, the time- relations of 

 physiological action are generally such that internal effects of 

 brief duration produce small external effects that cannot be 

 standardised by a prolonged external voltage. 



Thus, e.g., a nervous impulse with a duration of say 0.005 sec. 

 might, on a given instrument, produce the same deflection as a 

 constant current from an E.M.F. of o.ooi volt, but this would 

 not indicate electromotive value of the nervous impulse. A closer 

 approximation would be arrived at by making the comparison with 



