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 disposition :
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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.
