ence in the periods of recovery: (1) from instantaneous
E.M.F.; and (2) that caused by mechanical stimulus.]

The record was then taken as follows. The recording drum had a fast
speed of six inches in a minute, one of the small subdivisions
representing a second. The battery contact in the main potentiometer
circuit was made for a quarter of a second as just mentioned and a
record taken of the effect of a short-lived E.M.F. on the circuit
containing the cell. (2) A record was next taken of the E.M. variation
produced in the cell by a single stimulus. It will be seen on comparison
of the two records that the maximum effect took place relatively later
in the case of mechanical stimulus, and that whereas the galvanometer
recovery in the former case took place in 11 seconds, the recovery in
the latter was not complete till after 60 seconds (fig. 60, _b_). This
shows that it takes some time for the effect of stimulus to attain its
maximum, and that the effect does not disappear till after the lapse of
a certain interval. The time of recovery from strain depends on the
intensity of stimulus. It takes a longer time to recover from a stronger
stimulus. But, other things being equal, successive recovery periods
from successive stimulations of equal intensity are, generally speaking,
the same.

We may now study the influence of any change in external conditions by
observing the modifications it produces in the normal curve.

[Illustration: FIG. 61.--PROLONGATION OF PERIOD OF RECOVERY AFTER
OVERSTRAIN
Recovery is complete in 60" when the stimulus is due to 20 deg. vibration.
But with stronger stimulus of 40 deg. vibration, the period of recovery
is prolonged to 90".]

#Prolongation of period of recovery by overstrain.#--The pair of records
given in fig. 61 shows how recovery is delayed, as the effect of
overstrain. Curve (_a_) is for a single stimulus due to a vibration of
amplitude 20 deg., and curve (_b_) for a stimulus of 40 deg. amplitude of
vibration. It will be noticed how relatively prolonged is the recovery
in the latter case.

[Illustration: FIG. 62.--MODEL SHOWING THE EFFECT OF FRICTION]

#Molecular Model.#--We have seen that the electric response is an outward
expression of the molecular disturbance produced by the action of the
stimulus. The rising part of the response-curve thus exhibits the effect
of molecular upset, and the falling part, or recovery, the restoration
to equilibrium. The mecha