ANIMAL ELECTRICITY. LECTURE II. 2)1 



however, by compensation, and you may, if desired, 

 use the resistance boxes r R for this purpose. You 

 have only to see that the current from r opposes the 

 nerve current at the keyboard, and by successive trials 

 find that value of r + u. ^.t w^hich there is no current 

 through the galvanometer. The nerve current is then 

 compensated or balanced just as a given mass in one 

 scale-pan is balanced by an equal weight in another 

 scale-pan. Say, e.g., that the nerve current is balanced 

 when -fi^^ is J°^^ then — taking the pressure of the 

 Leclanche cell at i -5 volt — the pressure of your nerve 

 current is 0*015 volt. 



It has been customary to take observations of the 

 excitatory changes (negative variation, &c.) of nerve, 

 with such compensation in force, and for rheotome 

 observations this is necessary. But for observations 

 such as these it is a mistake to use a compensating 

 current ; in its presence we have opposing each other 

 in the nerve {a) the injury current from T to L ; {b) 

 the compensator current from L to T. and the latter 

 current has its anode at L. We shall find later that 

 a foreign (polarising) current increases the zincability 

 of living nerve at its point of entrance or anode. ^ 

 The negative deflection due to excitation and zinca- 

 tivity aroused at L is therefore unduly exaggerated — 

 reinforced by what we shall recognise as the polarisa- 

 tion increment of the compensating current. 



^ See below, the polarisation increment, p. 135. 



