INORGANIC RESPONSE--MODIFIED APPARATUS TO EXHIBIT RESPONSE IN METALS
Conditions of obtaining quantitative measurements--Modification of the
block method--Vibration cell--Application of stimulus--Graduation of
the intensity of stimulus--Considerations showing that electric
response is due to molecular disturbance--Test experiment--Molecular
voltaic cell.
We have already seen that metals respond to stimulus by E.M. variation,
just as do animal and vegetable tissues. We have yet to see whether the
similarity extends to this point only, or goes still further, whether
the response-curves of living and in organic are alike, and whether the
inorganic response-curve is modified, as living response was found to
be, by the influence of external agencies. If so, are the modifications
similar? What are the effects of superposition of stimuli? Is there
fatigue? If there be, in what way does it affect the curves? And lastly,
is the response of metals exalted or depressed by the action of chemical
reagents?
#Conditions of obtaining quantitative measurements.#--In order to carry
out these investigations, it is necessary to remove all sources of
uncertainty, and obtain quantitative measurements. Many difficulties at
first presented themselves in the course of this attempt, but they were
completely removed by the adoption of the following experimental
modification. In the simple arrangement for qualitative demonstration of
response in metals previously described, successive experiments will not
give results which are strictly comparable (1) unless the resistance of
the circuit be maintained constant. This would necessitate the adoption
of some plan for keeping the electrolytic contacts at A and B absolutely
invariable. There should then be no chance of any shifting or variation
of contact. (2) There must also be some means of applying successive
stimuli of equal intensity. (3) And for certain further experiments it
will be necessary to have some way of gradually increasing or decreasing
the stimuli in a definite manner.
#Modification of the block method.#--By consideration of the following
experimental modifications of the block method (fig. 55), it will be
found easy to construct a perfected form of apparatus, in which all
these conditions are fully met. The essentials to be kept in mind were
the introduction of a complete block midway in the wire, so that the
disturbance of one half should be prevented from reaching the other, and
the making of a perfect electrolytic contact for the electrodes leading
to the galvanometer.
Starting from the simple arrangement previously described where a
straight wire is clamped in the middle (fig. 55, _a_), we next arrive at
(_b_). Here the wire A B is placed in a U tube and clamped in the
middle by a tightly fitting cork. Melted paraffin wax is poured to a
certain depth in the bend of the tube. The two limbs of the tube are
now filled with water, till the ends A and B are completely immersed.
Connection is made with the non-polarisable electrodes by the side
tubes. Vibration may be imparted to either A or B by means of ebonite
clip holders seen at the upper ends A B of the wire.
[Illustration: FIG. 55.--SUCCESSIVE MODIFICATIONS OF THE BLOCK METHOD
FROM THE 'STRAIGHT WIRE' (_a_) TO 'CELL FORM' (_e_)
When A is excited, current of response _in the wire_ is from less
excited B to more excited A. Note that though the current of
response is constant in direction, the galvanometer deflection in
(_d_) will be opposite to that in (_b_).]
It will be seen that the two limbs of the tube filled with water serve
the purpose of the strip of moistened cloth used in the last experiment
to make electric connections with the leading-out electrodes--with the
advantage that we have here no chance of any shifting of contact or
variation of surface, the contact between the wire and the surrounding
liquid being perfect and invariable.
On now vibrating the end A of the tin wire by means of the ebonite clip
holder, a current will be found to flow from B to A through the
wire--that is to say, towards the excited--and from A to B in the
galvanometer.
The next modification (_c_) is to transfer the galvanometer from the
electrolytic to the metallic part of the circuit, that is to say, it is
interposed in a gap made by cutting the wire A B, the upper part of the
circuit being directly connected by the electrolyte. Vibration of A will
now give rise to a current of response which flows in the metallic part
of the circuit with the interposed galvanometer from B to A. We see that
though the direction of the current in this is the same as in the last
case, yet the galvanometer deflection is now reversed, for the evident
reason that we have it interposed in the metallic and not in the
electrolytic part of the circuit.
The next arrangement (_d_) consists simply of the preceding placed
upside down. Here A and B are held parallel to each other in an
electrolytic bath (water). Mechanical vibration may now be applied to A
without affecting B, and _vice versa_.
The actual apparatus, of which this is a diagrammatic representation, is
seen in (_e_).
Two pieces, from the same specimen of wire, are clamped separately at
their lower ends by means of ebonite screws, in an L-shaped piece of
ebonite. The wires are fixed at their upper ends to two
electrodes--leading to the galvanometer--and kept moderately and
uniformly stretched by spiral springs. The handle, by which a torsional
vibration is imparted to the wire, may be slipped over either electrode.
The amplitude of vibration is measured by means of a graduated circle.
It will be seen from these arrangements:
(1) That the cell depicted in (_e_) is essentially the same as that in
(_a_).
(2) That the wires in the cell being immersed to a definite depth in the
electrolyte there is always a perfect and invariable contact between the
wire and the electrolyte. The difficulty as regards variation of contact
is thus eliminated.
(3) That as the wires A and B are clamped separately below, we may
impart a sudden molecular disturbance to either A or B by giving a quick
to-and-fro (torsional) vibration round the vertical wire, as axis, by
means of the handle. As the wire A is separate from B, disturbance of
one will not affect the other. Vibration of A produces a current in one
direction, vibration of B in the opposite direction. Thus we have means
of verifying every experiment by obtaining corroborative and reversed
effects. When the two wires have been brought to exactly the same
molecular condition by the processes of annealing or stretching, the
effects obtained on subjecting A or B to any given stimulus are always
equal (fig. 56).
[Illustration: FIG. 56.--EQUAL AND OPPOSITE RESPONSES EXHIBITED BY A
AND B]
Usually I interpose an external resistance varying from one to five
megohms according to the sensitiveness of the wire. The resistance of
the electrolyte in the cell is thus relatively small, and the
galvanometer deflections are proportional to the E.M. variations. It is
always advisable to have a high external resistance, as by this means
one is not only able to keep the deflections within the scale, but one
is not troubled by slight accidental disturbances.
#Graduation of intensity of stimulus.#--If now a rapid torsional vibration
be given to A or B, an E.M. variation will be induced. If the amplitude
of vibration be kept constant, successive responses--in substances
which, like tin, show no fatigue--will be found to be absolutely
identical. But as 'the amplitude of vibration' is increased, response
will also become enhanced (see Chap. XV).
[Illustration: FIG. 57.--TOP VIEW OF THE VIBRATION CELL
The amplitude of vibration is determined by means of movable stops S S',
fixed to the edge of the graduated circle G. The index arm I plays
between the stops. (The second index arm, connected with B, and the
second circle are not shown.)]
Amplitude of vibration is measured by means of the graduated circle
(fig. 57). A projecting index, in connection with the vibration-head,
plays between fixed and sliding stops (S and S'), one at the zero point
of the scale, and the other movable. The amplitude of a given vibration
can thus be predetermined by the adjustment of the sliding stop. In this
way we can obtain either uniform or definitely graduated stimuli.
#Considerations showing that electric response is due to molecular
disturbance.#--The electromotive variation varies with the substance.
With superposition of stimuli, a relatively high value is obtained in
tin, amounting sometimes to nearly half a volt, whereas in silver the
electromotive variation is only about ·01 of this value. The intensity
of the response, however, does not depend on the chemical activity of
the substance, for the electromotive variation in the relatively
chemically inactive tin is greater than that of zinc. Again, the sign of
response, positive or negative, is sometimes modified by the molecular
condition of the wire (see Chap. XII).
As regards the electrolyte, dilute NaCl solution, dilute solution of
bichromate of potash &c. are normal in their action, that is to say, the
electric response in such electrolytes is practically the same as with
water. Ordinarily I use tap-water as the electrolyte. Zinc wires in
ZnSO_4 solution give responses similar in character to those given by,
for example, Pt or Sn in water.
#Test experiment.#--It may be urged that the E.M. effect is due in some
way (1) to the friction of the vibrating wire against the liquid; or (2)
to some unknown surface action, at the point in the wire of the contact
of liquid and air surfaces. This second objection has already been
completely met in experimental modification, fig. 55, _b_, where the
wire was shown to give response when kept completely immersed in water,
variation of surface being thus entirely eliminated.
Both these questions may, however, be subjected to a definite and final
test. When the wire to be acted on is clamped below, and vibration is
imparted to it, a strong molecular disturbance is produced. If now it be
carefully released from the clamp, and the wire rotated backwards and
forwards, there could be little molecular disturbance, but the liquid
friction and surface variation, if any, would remain. The effect of any
slight disturbance outstanding owing to shaking of the wire would be
relatively very small.
We can thus determine the effect of liquid friction and surface action
by repeating an experiment with and without clamping. In a tin wire
cell, with interposed external resistance equal to one million ohms, the
wire A was subjected to a series of vibrations through 180°, and a
deflection of 210 divisions was obtained. A corresponding negative
deflection resulted on vibrating the wire B. Now A was released from the
clamp, so that it could be rotated backwards and forwards in the water
by means of the handle. On vibrating the wire A no measurable deflection
was produced, thus showing that neither water friction nor surface
variation had anything to do with the electric action. The vibration of
the still clamped B gave rise to the normal strong deflection.
As all the rest of the circuit was kept absolutely the same in the two
different sets of experiments, these results conclusively prove that
the responsive electro-motive variation is solely due to the molecular
disturbance produced by mechanical vibration in the acted wire.
A new and theoretically interesting molecular voltaic cell may thus be
made, in which the two elements consist of the _same metal_. Molecular
disturbance is in this case the main source of energy. A cell once made
may be kept in working order for some time by pouring in a little
vaseline to prevent evaporation of the liquid.
It will be shown further, in succeeding chapters, by numerous instances,
that any conditions which increase molecular mobility will also increase
intensity of response, and conversely that any conditions having the
reverse effect will depress response.