Part 9 (1/2)
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.
[Ill.u.s.tration: FIG. 60 (_a_) Arrangement for applying a short-lived E.M.F.
(_b_) Difference 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.
[Ill.u.s.tration: FIG. 61.--PROLONGATION OF PERIOD OF RECOVERY AFTER OVERSTRAIN Recovery is complete in 60” when the stimulus is due to 20 vibration.
But with stronger stimulus of 40 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, and curve (_b_) for a stimulus of 40 amplitude of vibration. It will be noticed how relatively prolonged is the recovery in the latter case.
[Ill.u.s.tration: 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 mechanical model (fig. 62) will help us to visualise many complex response phenomena. The molecular model consists of a torsional pendulum--a wire with a dependent sphere. By the stimulus of a blow there is produced a torsional vibration--a response followed by recovery. The writing lever attached to the pendulum records the response-curves. The form of these curves, stimulus remaining constant, will be modified by friction; the less the friction, the greater is the mobility. The friction may be varied by more or less raising a vessel of sand touching the pendulum. By varying the friction the following curves were obtained.
(_a_) When there is little friction we get an after-oscillation, to which we have the corresponding phenomenon in the retinal after-oscillation (compare fig. 105).
(_b_ and _c_) If the friction is increased, there is a damping of oscillation. In (_c_) we get recovery-curves similar to those found in nerve, muscle, plant, and metal.
(_d_) If the friction is still further increased the maximum is reached much later, as will be seen in the increasing slant of the rising part of the curve; the height of response is diminished and the period of recovery very much prolonged by partial molecular arrest. The curve (_d_) is very similar to the 'molecular arrest' curve obtained by small dose of chemical reagents which act as 'poison' on living tissue or on metals (compare fig. 93, _a_).
(_e_) When the molecular mobility is further decreased there is no recovery (compare fig. 93, _b_).
Still further increase of friction completely arrests the molecular pendulum, and there is no response.
From what has been said, it will be seen that if in any way the friction is diminished or mobility increased the response will be enhanced. This is well exemplified in the heightened response after annealing (fig. 58) and after preliminary vibration (figs. 81, 82).
Possibly connected with this may be the increased responses exhibited by the action of stimulants (figs. 89, 90).
#Reduction of molecular sluggishness attended (1) by quickened recovery.#--Sometimes, after a cell has been resting for too long a period, especially on cold days, the wire gets into a sluggish condition, and the period of recovery is thereby prolonged. But successive vibrations gradually remove this inertness, and recovery is then hastened. This is shown in the accompanying curves, fig. 63, where (_a_) exhibits only very partial recovery even after the expiration of 60 seconds, whereas when a few vibrations had been given recovery was entirely completed in 47 seconds (_b_). There was here little change in the height of response.
[Ill.u.s.tration: FIG. 63 (_a_) Slow recovery of a wire in a sluggish condition.
(_b_) Quickened recovery in the same wire after a few vibrations.]
#Or (2) by heightened response.#--The removal of sluggishness by vibration, resulting in increased molecular mobility, is in other instances attended by increase in the height of response, as will be seen from the two sets of records which follow (fig. 64). Cold, due to prevailing frosty weather, had made the wires in the cell somewhat lethargic. The records in (_a_) were the first taken on the day of the experiment. The amplitudes of vibration were 45, 90, and 135. In (_b_) are given the records of the next series, which are in every case greater than those of (_a_). This shows that previous vibration, by conferring increased mobility, had heightened the response. In this case, removal of molecular sluggishness is attended by greater intensity of response, without much change in the period of recovery. In connection with this it must be remembered that greater strain consequent on heightened response has a general tendency to a prolongation of the period of recovery.
[Ill.u.s.tration: FIG. 64 (_a_) Three sets of responses for 45, 90, and 135 vibration in a sluggish wire.
(_b_) The next three sets of responses in the same wire; increased mobility conferred by previous vibration has heightened the response.]
It is thus seen that when the wire is in a sluggish condition, successive vibrations confer increased molecular mobility, which finds expression in quickened recovery or heightened response.
#Effect of temperature.#--Similar considerations lead us to expect that a moderate rise of temperature will be conducive to increase of response.
This is exhibited in the next series of records. The wire at the low temperature of 5 C. happened to be in a sluggish condition, and the responses to vibrations of 45 to 90 in amplitude were feeble. Tepid water at 30 C. was now subst.i.tuted for the cold water in the cell, and the responses underwent a remarkable enhancement. But the excessive molecular disturbance caused by the high temperature of 90 C. produced a great diminution of response (fig. 65).