position, alternate equal stimulations of A and B produce equal and opposite electromotive effects, and when the two ends are stimulated simultaneously there is no resultant effect.
Increased Excitability produced by Preliminary Vibration—If now one-half of the wire, say the A half, be vibrated for a time, the electric excitability of that half will be found to be more or less permanently augmented, presumably by increased molecular mobility conferred by vibration. The response of A would now be found to be very much enhanced, as compared with its previous response, the response of B remaining the same as before. If now both the ends are simultaneously vibrated, the balance which previously existed will be found to be disturbed, the resultant showing that A has been rendered the more excitable.
If B be now vibrated for a time, the former approximate balance will be re-established by the enhanced responsiveness of B. Thus in the following experiment with the clamp at the approximate balancing point—
| Response of A. | Response of B. | Resultant response. | ||||
| Approximate balance | + 5 | divisions | -4.5 | divisions | +0.5 | divisions |
| After the end A has been vibrated | +10.5 | ,, | -4.5 | ,, | +6.0 | ,, |
| After the end B has been vibrated for an equal length of time | +10.5 | ,, | -9.5 | ,, | +1.0 | ,, |
Effect of Chemical Reagents. It will be shown that keeping the electrolyte by which contact is made constant, the electric excitability of the wire depends on the molecular condition of the wire. Certain electrolytes, such as dilute solution of NaCl, dilute solution of bichromate of potash, &c., are normal in their action, that is to say, with such contacts the response to stimulation is practically the same as with distilled water contact.
Contact made with dilute NaCl solution may therefore be regarded as the normal contact. There are again certain chemical reagents which enhance the electrical excitability; others on the contrary produce great depression, or abolition of excitability.
Electric Comparator
We may compare the relative electric excitability conferred by chemical reagents by the method of balance. Having previously obtained a balance (with water or dilute NaCl solution contacts
