140 ANIMAL ELECTRICITY. LECTURE VI. 



Here is the experiment of the polarisation incre- 

 ment with a sHghtly different circuit. The polarising 

 current, galvanometer, secondary coil and nerve are 

 in one circuit. The nerve receives the polarising 

 and exciting currents, and gives its electrical response 

 through a single pair of unpolarisable electrodes. 



The first thinor to do is to see that there is little 

 or no current from the electrodes and portion of nerve 

 between— if the nerve happened to be injured near 

 one or other of the electrodes, we should have a 

 current of injury and on subsequent excitation a 

 negative variation of that current that might confuse 

 or mislead us. 



The next step is to set the alternator in movement 

 and see that the induction shocks rapidly pulsating 

 to and fro through the nerve and galvanometer, do 

 not of themselves give any deflection. If the induc- 

 tion currents were taken too strong, we should be 

 confused by an initial and terminal kick by the first 

 make and last break shock of the exciting series, and 

 by a permanent deflection during the series in the 

 direction of the break current, to which allusion was 

 made above (p. 114). 



These two preliminary sources of error having 

 been excluded, we may proceed with the experiment 

 proper. 



I close a key in the polarising circuit. The spot 

 flies off to the right. I bring it back to scale by 

 means of the controllinijf maonet. And now that it 



