e application of stimulus, and completing it at various
short intervals after the cessation, when a persisting electrical
effect, diminishing rapidly with time, will be apparent. The rate of
recovery immediately on the cessation of stimulus is rather rapid, but
traces of strain persist for a short time.

CHAPTER XIII

INORGANIC RESPONSE--MOLECULAR MOBILITY: ITS INFLUENCE ON RESPONSE

Effects of molecular inertia--Prolongation of period of recovery by
overstrain--Molecular model--Reduction of molecular sluggishness
attended by quickened recovery and heightened response--Effect of
temperature--Modification of latent period and period of recovery by
the action of chemical reagents--Diphasic variation.

We have seen that the stimulation of matter causes an electric
variation, and that the acted substance gradually recovers from the
effect of stimulus. We shall next study how the form of response-curves
is modified by various agencies.

In order to study these effects we must use, in practice, a highly
sensitive galvanometer as the recorder of E.M. variations. This
necessitates the use of an instrument with a comparatively long period
of swing of needle, or of suspended coil (as in a D'Arsonval). Owing to
inertia of the recording galvanometer, however, there is a lag produced
in the records of E.M. changes. But this can be distinguished from the
effect of the molecular inertia of the substance itself by comparing two
successive records taken with the same instrument, in one of which the
latter effect is relatively absent, and in the other present. We wish,
for example, to find out whether the E.M. effect of mechanical stimulus
is instantaneous, or, again, whether the effect disappears immediately.
We first take a galvanometer record of the sudden introduction and
cessation of an E.M.F. on the circuit containing the vibration-cell
(fig. 60, _a_). We then take a record of the E.M. effect produced by a
stimulus caused by a single torsional vibration. In order to make the
conditions of the two experiments as similar as possible, the disturbing
E.M.F., from a potentiometer, is previously adjusted to give a
deflection nearly equal to that caused by stimulus. The torsional
vibration was accomplished in a quarter of a second, and the contact
with the potentiometer circuit was also made for the same length of
time.

[Illustration: FIG. 60
(_a_) Arrangement for applying a short-lived E.M.F.
(_b_) Differ