of light. The disturbance of molecular equilibrium caused by the stimulus is attended by an electromotive variation, which gradually disappears on the restoration of the molecules to equilibrium. The rising portion of the response curve shows the electromotive effect, due to stimulus, and the falling portion the recovery. The ordinate represents the electromotive variation, and the abscissa the time.
Experiments to Exhibit the Balancing Effect.
If the wire has been carefully annealed, the molecular conditions of different portions are approximately the same. Every portion of the surface will be found nearly iso-electric. If the wire be held near the middle by the clamp, and a vibration through an amplitude of, say, 90° be given to the end A, an upward deflection will be produced; if a vibration of 90° be given to B, there will be produced an equal downward deflection. (Fig. 1, c.) If both the ends are simultaneously vibrated, the electromotive Variation at the two ends will continuously balance each other, and the galvanometer spot will remain quiescent. (Fig. 1, d.) The clamp may even be removed, and the wire vibrated as a whole; the stimulation of A and B being the same, there will be no resultant deflection. Having found the balancing point for the clamp (which is at or near the middle), if the clamp be now shifted to the left, on simultaneous vibration of A and B, the A effect will be relatively stronger (inasmuch as the angular vibration of A is increased and that of B decreased), and there will be produced a resultant upward deflection. Thus keeping the rest of the circuit untouched, by merely moving the clamp from the left, past the balancing position to the right we get either a positive or zero or a negative resultant effect. This can be repeated any number of times. The experiment shows further that when the amplitude of vibration is kept constant, the intensity of electromotive effect is increased by shortening the wire. A thick wire produces a stronger response. The direction of the current of response in the wire is in the majority of metals under normal condition, from the relatively less to the relatively more excited point.
The form of the response curve, stimulus remaining constant, is modified by the molecular condition of the wire. A wire in a sluggish condition shows feeble response, the recovery is also slow. The same wire after it has been vibrated for a time exhibits stronger response. The period of recovery may also then be hastened. Longer time is required for recovery from the effects of a stronger stimulus.
Comparison of Electric Excitability of Two Points by the Method of Balance.
As has already been said, when the clamp is put at the balancing
