a potentiometer the galvanometer spot may be brought back to the original position.) The shifting of the zero will not affect the general result. The direction of this more or less permanent current, due to the small P.D., gives no indication of the direction of current of response; the direction of the latter is determined by the rule that the responsive current flows towards the more excitable point. The effect of the mechanical stimulus is to produce a transient electromotive force which is algebraically superposed on the existing P.D. The deflection will take place from the modified zero, to which the spot returns during recovery. I give four records (fig. 2): in (a) A is touched with Na2CO3 (which is an excitant): a permanent current flows from B to A: response to stimulus is in the same direction as the permanent current (positive variation); in (b) A is touched with a trace of oxalic acid (which depresses the excitability), the permanent current is in the same direction as before, but the current of response is in the opposite direction (negative variation); in (c) A is touched with dilute KHO (3 parts in 1000), the response is exhibited by a positive variation; in (d) A is touched with stronger KHO parts in 100), the response is now exhibited by a negative variation. The last two apparently anomalous results are due to the fact (which will be demonstrated later) that KHO in minute quantities is an excitant, while in larger quantities it is a depressant.
Fig. 2.
(a) Response when A is treated with sodium carbonate—an apparent positive variation
(b) ,,,,,,,,,, oxalic acid—an apparent negative variation
(c) ,,,,,,,,,, very dilute potash—positive variation.
(d) ,,,,,,,,,, strong potash—negative variation.
This response is up when A is more excitable and down when B is more excitable.
Lines thus — — — indicate direction of permanent current.
