at A and B), one contact, say A, is touched with a few drops of Na2CO3, which is an exciting agent. The electric excitability of A will now be found to be greater than that of B; on simultaneous vibration of A and B there will be a disturbance of balance, giving rise to an upward deflection (current of response towards the more excitable A).
| Response of A. | Response of B. | Resultant Response. | |
| Both contacts of normal saline | +12 | -12 | 0 |
| Contact A touched with Na2CO3 solution | + 32 | -12 | +20 |
Similarly, when A is depressed by a trace of oxalic acid the electric excitability of A is less than that of B, the resultant deflection being now downwards (current of response towards the relatively more responsive B). It is to be remembered that in all cases the resultant current of response the wire is towards the more excitable point.
An interesting line of investigation rendered possible by a modification of method of balance described above is to compare the relative excitability induced by various chemical reagents, the influence of the same reagent of different strengths, and the modification of the effect caused by the duration of application. We may thus compare the effect of the reagent in relation to the normal effect of water or dilute NaCl solution. There is again an extremely delicate method of comparison of the relative effects of a series of compounds like Na2CO3, K2CO3, &c. Balance having been previously obtained between the normal sensitiveness of A and B, the two different solutions are now applied at the two points; the slightest difference in their relative action is at once exhibited by. the upsetting of the balance during stimulation, the direction of the resultant deflection indicating the more exciting reagent.
Resultant Response by Method of Relative Depression of Exaltation.
From What has been said, it will be seen that by rendering A and B unequally excitable, a resultant response may be obtained. The block may be abolished, and the wire may be vibrated as a whole; the response will now be due to the differential effect at A and B. To produce difference in excitability we may subject one point, say A, to a preliminary vibration, or apply at the point a suitable chemical reagent. By the application of the latter there will be a small P.D. between A and B: this will simply produce a displacement of the zero. (By means of
