80 STUDIES IN ELECTRO-PHYSIOLOGY: 



Similarly the wire of 0-02 in. section with a resistance 

 of 0-0884 ohm per metre would give us 8,840 ohms total 

 resistance and 1,360 ohms per ft. length, and while this 

 brings us no nearer to the actual resistance of a nerve, it 

 approximates somewhat to the resistance of the hand-to- 

 hand circuit, in which, by reason of the absence of sebaceous 

 glands in the palms of the hands, skin resistance is much 

 lower than in most other parts of the body. 



This conclusion is arrived at in the following manner : 



Upon the scale of a reflecting galvanometer which has a 

 sensibility of 4,000 mm., at a metre distance from the scale, 

 per micro-ampere, the average hand-to-hand deflection 01 

 a person in normal health is between 300 and 400 mm., 

 equivalent to a current of from 0-08 to 0-1 micro-ampere. 



The mean of several thousands of tests has shown the 

 electromotive force of man to range between 4 and 5 

 millivolts, and, as C = ^, we can, knowing C and E, cal- 

 culate R with some approach to accuracy. By this 

 method we should find the resistance of the hand-to-hand 

 circuit to be over 5,000 ohms, taking into considera- 

 tion the difference of sensibility or response to current 

 and voltage. The calculation, however, is not given with 

 the confidence that would attach to a bridge test in 

 which the natural current was used, to the exclusion of 

 battery power. 



5,000 ohms would be lower by 3,840 ohms, or 590 ohms 

 per ft. length, than the wire of 0-02 in. sectional area, but 

 in the circuit in question there are several conductors, and 

 among them the main leads of the thumbs. 



The resistance of nerves, whatever may be their 

 expression in ohms, must vary in many parts of the body, 

 and, irrespective of the surface area of the conducting 

 plates or discs or rods of the body condensers, have the 

 effect of altering capacity ; while further variations are 

 introduced by the inconstancy of the human electro- motive 



