182 Dr. A. D. Waller. 



and of variable direction. Two outgoing currents are aroused at the 

 points A and B, of opposed directions in the circuit, and the deflection 

 under these conditions is only resultant of an inequality in the two 

 forces aroused at A arid at B. 



In order to investigate separately the local reactions of A and of B 

 consequent upon electrical excitation, of which these two points are 

 poles, it is necessary to employ a third electrode, C, in the following 

 manner : 



B and A are the exciting electrodes, C is an independent electrode 

 through which no exciting current is passed, and which in conjunction 

 Avith A or with B is to serve as leading-off' electrode to the galvano- 

 meter. 



If it be intended to examine the state at A after passage of an 

 exciting current through A B, the procedure will be : 1. Compensa- 

 tion of the two points A and C. 2. Excitation through A B. 3. Con- 

 nection of A and C with the galvanometer. 



If it be intended to examine the state at B, the similar procedure 

 will be : 1. Compensation of B and C. 2. Excitation through A B. 

 3. Connection of B and C with the galvanometer. 



The necessary transposition from A to C or from B to C is most 

 readily effected by means of a switch key of the following disposi- 

 tion : 



:\\ 



B C A B C A B C A 



The three electrodes are connected with the terminals A, B, C. A 

 is connected with A' and B with B' by revolving arms, either of whjch 

 can be turned on to the middle terminal C. 



This method is applicable to many cases other than that of the skin. 

 I shall have occasion to refer to it as the " A, B, C Method," and for 

 the sake of brevity shall call the above-described switch the " M 

 switch." 



The following protocol will best exhibit the results of a typical 

 experiment and which is a point of some practical importance the 

 system on which laboratory notes are taken so as to avoid confusion 

 of direction during experiment. 



Experiment 9. Fiw/'s Skin, excited by Single Condenser Discluirges. 

 Three electrodes, A, B, and C, applied to external surface. Excitation 

 led in through A B. Response led off through A C, and B C, outgoing 

 currents at A and at B. 



