Galvanometer throws. Galvanometer throws.
1st closing 1° 1st opening 18°
2nd ” 64° 2nd ” 100°
3rd ” 146° 3rd ” 140°
_Effects of Passing Continuous Currents of High E.M.F._--If a
continuous current of high E.M.F. is employed, it renders a sensitive
substance conducting. The phenomenon may be shown in the following
manner. A circuit is made up consisting of a battery, a sensitive
substance, and a galvanometer. The E.M.F. of the battery is first one
volt, then 100 volts, then one volt. Below I give the galvanometer
deflections obtained with an E.M.F. of one volt for three different
substances before and after the application of the E.M.F. of 100
volts:--
Before application of current. After application of current.
16 100
0 15
1 500
In the case of some measurements taken on a Wheatstone bridge a prism
of aluminium filings interposed between two copper electrodes offered
a resistance of several million ohms before a high E.M.F. was applied,
but only offered a resistance of 350 ohms after the application of
this pressure for one minute. The time during which the powder should
be interposed in the battery circuit should not be too short. Thus, in
one instance, the application for 10 sec. of 75 mercury sulphate cells
produced no effect, but their application for 60 sec. resulted in the
resistance being reduced from several megohms to 2,500 ohms.
It should be observed that the phenomenon of suddenly increased
conductivity occurs, even if the sensitive substance is not in circuit
with a battery at the time it is influenced. Thus, the metallic
filings, after having been placed in circuit with a Daniell cell, and
its high resistance observed, may then be completely insulated and
submitted in this condition to the action of a distant spark, or of a
charged rod, or of induced currents. If, after this, the filings are
replaced in their original circuit, the enormous increase in their
conductivity is immediately apparent.
The conductivity produced by these various methods takes place
throughout the whole mass of the metallic filings, and in every
direction, as the following experiment will show. A vertical ebonite
cup containing aluminium powder (_see_ Fig. 54) is placed between two
metal plates, A, B; laterally the powder is in contact with two short
rods, C, D, which pass through the sides of the ebonite cylinder. A
and B can be connected to two terminals of one of the arms of the
Wheatstone bridge, C and D being free, and _vice versâ_. Whatever
arrangement is adopted, if a battery of 100 cells is joined up for
a few seconds with one or the other of the pairs of terminals, the
increase in the conductivity is immediately visible in that direction,
and is found to exist also in the direction at right angles.
_Substances in which Diminution of Resistance has been Observed._--The
substances in which the phenomenon of the sudden increase of
conductivity is most easily observed are filings of iron, aluminium,
copper, brass, antimony, tellurium, cadmium, zinc, bismuth, &c.
The size of the grains and their nature are not the only elements to
be considered, for grains of lead of the same size, but coming from
different quarters, offer at the same temperature great differences in
resistance (20,000 to 500,000 ohms). Extremely fine metallic powder, as
a rule, offers almost perfect resistance to the passage of a current.
But if we take a sufficiently short column and exert a sufficiently
great pressure a point is soon reached when the electrical influence
will effect a sudden increase in the conductivity. Thus, a layer of
copper reduced by hydrogen, which does not become conducting under the
influence of the electric spark or otherwise, will become so on being
submitted to a pressure of 500 grammes to the square centimetre (7 lb.
per square inch). Instead of using pressure, I employed as a conductor
in some experiments a very fine coating of powdered copper spread on
a sheet of unpolished glass or ebonite E (Fig. 55), seven centimetres
long and two centimetres broad. A layer of this kind, polished with a
burnisher, has a very variable resistance. With a little care one can
prepare sheets which are more or less sensitive to electrical action.
[Illustration: FIG. 55.]
Metal powder or metal filings are not the only sensitive substances, as
powdered galena, which is slightly conducting under pressure, conducts
much better after having been submitted to electrical influence.
Powdered binoxide of maganese is not very sensitive unless mixed with
powdered antimony and compressed.
Making use of the A arrangement, with very short sparks at S (Fig.
35), the phenomenon of increased conductivity can be observed with
platinised and silvered glass, also with glass covered with gold,
silver and aluminium foil. Some of the mixtures employed had the
consistency of paste. These were mixtures of colza oil and iron,
or antimony filings, and of ether or petroleum and aluminium, and
plumbago, &c. Other mixtures were solid. If we make a mixture
of iron filings and Canada balsam, melted in a water bath, and pour
the paste into a little ebonite cup, the ends of which are closed by
metallic rods, a substance is obtained which solidifies on cooling. The
resistance of such a mixture is lowered from several megohms to a few
hundred ohms by an electric spark. Similar results are obtained with a
solid rod composed of fused flowers of sulphur and iron or aluminium
filings, also by a mixture of melted resin and aluminium filings. In
the preparation of these solid sensitive mixtures care must be taken
that the insulating substance should only form a small percentage of
the whole.
Some interesting results are also obtained with mixtures of sulphur and
aluminium, and with resin and aluminium, when in a state of powder.
When cold, these mixtures as a rule do not conduct either directly
or after they have been exposed to electrical influences, but they
become conducting on combining pressure with electrical influences.
Thus, a mixture of flowers of sulphur and aluminium filings in equal
volumes was placed in a glass tube 24 mm. in diameter. The weight of
the mixture was 20 grammes, and the height of the column 22 mm.; with
a pressure of 186 grammes per square centimetre (2½ lb. per square
inch). The mixture is not conducting, but after exposure to electrical
influence, obtained by the A arrangement, the resistance falls to 90
ohms. In a similar manner a mixture of selenium and aluminium, placed
in a tube 99 mm. long, was not conducting until after it was exposed to
the combined influence of pressure and electricity.
The following is one of the group of numerous experiments of a slightly
different character. A mixture of flowers of sulphur and fine aluminium
filings, containing two of sulphur to one of aluminium, is placed in a
cylindrical glass tube 35 mm. long. By means of a piston, a pressure
of 20 kilogrammes per square centimetre (284 lb. per square inch) was
applied. It was only necessary to connect the column for 10 sec. to the
poles of a 25 cell battery, for the resistance originally infinite to
be reduced to 4,000 ohms.
The arrangement shown in Fig. 56 illustrates another order of
experiment. Two rods of copper were oxidised in the flame of a Bunsen
burner, and were then arranged to lie across each other, as shown, and
were connected to the terminals of the arm of a Wheatstone bridge,
the high resistance of the circuit being due to the layers of oxide.
Amongst the many measurements made, I found, in one case, a resistance
of 80,000 ohms, which, after exposure to the influence of the electric
spark, was reduced to 7 ohms. Analogous effects are obtained with
oxidised steel rods. Another pretty experiment is to place a cylinder
of copper, with an oxidised hemispherical head, on a sheet of oxidised
copper. Before exposure to the influence of the electric spark, the
oxide offers considerable resistance. The experiment can be repeated
several times by merely moving the cylinder from one place to another
on the oxidised sheet of copper, thus showing that the phenomenon only
takes place at the point of contact of the two layers of oxide.
[Illustration: FIG. 56.]
In conclusion, it may be worth noting that, for most of the substances
enumerated, an elevation of temperature diminishes the resistance, but
the effect of a rise of temperature is transient, and is incomparably
less than the effect due to currents of high potential. For a few
substances the two effects are opposed.
* * * * *
A second article by Mr. Branly in _La Lumière Electrique_ was
abstracted in _The Electrician_ for August 21, 1891, as follows:--
In a preceding article I showed that certain substances undergo an
increase in conductivity under various electrical influences, and that
these substances are numerous. The increase in conductivity varies with
the energy of the exciting source. If the electric influence is due to
the passage of a continuous current, the increase in conductivity is
greater the greater the electromotive force of the battery employed.
There is, however, no proportionality, the increased conductivity
growing more rapidly than the number of cells, and tending quickly to
a maximum. If the electric action consists in the passage of discharge
currents in metallic rods, as in arrangement A (Fig. 53, p. 97), the
conductivity increases with the length of spark at S, and it also
increases when the rods are brought nearer the sensitive substance.
Successive sparks are additive in their effects, although, if the
action of the first one has been very powerful, the resistance is
sometimes almost immediately reduced to a minimum.
_Restoration of Original Resistance._--The conductivity causes by the
various electrical influences lasts sometimes for a long period (24
hours or more), but it is always possible to make it rapidly disappear,
particularly by a shock.
The majority of substances tested showed an increase of resistance on
being shaken previous to being submitted to any special electrical
influence, but after having been influenced the effect of shock is much
more marked. The phenomenon is best seen with the metallic filings, but
it can also be observed with metalised ebonite sheets with mixtures of
liquid insulators and metallic powders, mixtures of metallic filings
and insulating substances (compressed or not compressed), and finally
with solid bodies.
I observed the return to original resistance in the following
manner:--The sensitive substance was placed at K (Fig. 53), and formed
part of a circuit which included a Daniell cell and galvanometer. At
first no current passes. Sparks are then caused at S, and the needle
of the galvanometer is permanently deflected. On smartly tapping the
table supporting the ebonite cap in which the sensitive substance is
contained, the original condition is completely restored. When the
electric action has been of a powerful character, violent blows are
necessary. I employed for the purpose of these shocks a hammer fixed on
the table, the blows of which could be regulated.
With some substances, when feebly electrified, the return seemed to be
spontaneous, although it was slower than the return of the galvanometer
needle to equilibrium. This restoration of the original resistance is
attributable to surrounding trepidations, as it was only necessary
to walk about the room at a distance of a few metres, or to shake a
distant wall. This spontaneous return to original resistance after weak
electrical action was visible with a mixture of equal parts of fine
selenium and tellurium powders. The restoration of resistance by shock
was not observable so long as the electrical influence was at work.
After having been submitted to powerful electric action, shock does not
seem to entirely restore substances to their original state, in fact,
the substances generally show greater sensitiveness to electric action.
Thus, a mixture of colza oil and antimony powder being exposed to the
influence of arrangement A, a spark of 5 mm. was at first necessary to
break down the resistance, but after the conductivity had been made to
disappear by means of blows, a spark of only 1 mm. was sufficient to
again render the substance conducting. Finely powdered aluminium has
an extremely high resistance. A vertical column of powdered aluminium
5 mm. long of 4 sq. cms. cross-section, submitted to considerable
pressure, completely stopped the current from a Daniell cell. The
influence of arrangement A produced no effect, but, by direct contact
with a Leyden jar, the resistance was reduced to 50 ohms. The effect of
shock was then tried, and after this the sparks produced by arrangement
A were able to reduce the resistance.
The following experiment is also of the same kind. Aluminium filings
placed in a parallelipidic trough completely stopped the current from
a Daniell cell, and the resistance offered to a single cell remained
infinite after the trough had been placed in the circuit of 25 sulphate
of mercury cells for 10sec. The aluminium was next placed in circuit
with a battery of 75 cells; a single Daniell cell was then able to send
a current through the substance. The original resistance was restored
by shock, but not the original condition of things, since a single
cell was able to send a current after the aluminium had been circuited
for 10 sec. with a battery of only 25 cells. I may add that if the
restoration of resistance was brought about by a violent shock, it
was necessary to place the aluminium in circuit with 75 cells for one
minute before the resistance was again broken down.
It must be observed that electrical influence is not always necessary
to restore conductivity after an apparent return to the original
resistance, repeated feeble blows being sometimes successful in
bringing this about. Both in the case of slow return by time and
sudden return by shock, the original value of the resistance is often
increased. Rods of Carré carbon, 1 metre long and 1 mm. in diameter,
were particularly noticeable for this phenomenon.
_Return to Original Resistance by Temperature Elevation._--A plate of
coppered ebonite rendered conducting by electricity, and placed close
to a gas jet, quickly regained its original resistance. A solid rod of
resin and aluminium, or of sulphur and aluminium, rendered conducting
by connection to the poles of a small battery will regain its original
resistance by shock; but if the conducting state has been caused by
powerful means, such, for instance, as direct contact with a Leyden
jar, shock no longer has any effect, at least such a shock as the
fragile nature of the material can stand. A slight rise of temperature,
however, has the desired result. By suitably regulating the electric
action it is possible to get a substance into such a condition that the
warmth of the fingers suffices to annul conductivity.
_Influence of Surroundings._--Electric action gives rise to no
alteration of resistance when the substance is entirely within a closed
metal box. The sensitive substance, in circuit with a Daniell cell and
a galvanometer, is placed inside a brass box (Fig. 54, p. 97). The
absence of current is ascertained, the circuit broken, and the box
closed. A Wimshurst machine is then worked a little way off, and will
be found to have had no effect. The same result will be obtained if
the circuit is kept closed during the time the Wimshurst machine is in
operation. If a wire connected at some point to the circuit is passed
out through a hole in the box to a distance of 20 cm. to 50 cm., the
influence of the Wimshurst machine makes itself felt. On tapping the
lid to restore resistance the galvanometer needle remains deflected
so long as the sparks continue to pass. If, however, the wires are
pushed in so that they only project a few millimetres, the sparks
still passing, a few taps suffice to bring back the needle to zero.
On touching the end of the wire with the fingers or a piece of metal
conductivity is immediately restored. The movements of the galvanometer
needle were rendered visible in these experiments by looking through a
piece of wide mesh wire-gauze with a telescope. The respective position
of the things was also reversed; that is to say, a Ruhmkorff coil
and a periodically discharged Leyden jar were placed inside, and the
sensitive substance outside, the box, with the same results.
In some later experiments with a larger metallic case (Fig. 57), and
with the Daniel cell, sensitive substance, delicate galvanometer, and
Wheatstone Bridge placed inside, I found that a double casing was
necessary in order to absolutely suppress all effects. A glass covering
afforded no protection.
[Illustration: FIG. 57.]
_Considerations on the Mechanism of the Effects Produced._--What
conclusions are we to draw from the experiments described? The
substances employed in these investigations were not conductors, since
the metallic particles composing them were separated from each other in
the midst of an insulating medium. It was not surprising that currents
of high potential, and especially currents induced by discharges,
should spark across the insulating intervals. But as the conductivity
_persisted_ afterwards, even for the weakest thermo-electric currents,
there is some ground for supposing that the insulating medium is
transformed by the passage of the current, and that certain actions,
such as shock and rise of temperature, bring about a modification of
this new state of the insulating body. Actual movement of the metallic
particles cannot be imagined in experiments where the particles, in a
layer a few millimetres thick, were fixed in an invariable relative
position by extreme pressures, reaching at times to more than 100
kilogrammes per sq. cm. (1,420 lb. to the square inch). Moreover, in
the case of solid mixtures, in which the same variations of resistance
were produced, displacement seems out of the question. To explain the
persistence of the conductivity after the cessation of the electrical
influence, are we to suppose in the case of metallic filings a partial
volatilisation of the particles creating a conducting medium between
the grains of metal? In the case of mixtures of metallic powders, and
insulating substances agglomerated by fusion, are we to suppose that
the thin insulating layers are pierced by the passage of very small
sparks, and that the holes left behind are coated with conducting
material? If this explication is admissible for induced currents, it
must hold good for continuous currents. If so, we must conclude that
these mechanical actions may be produced by batteries of only 10 to
20 volts electromotive force, and which only cause an insignificant
current to pass. The following experiment is worth quoting in this
connection:--
A circuit was formed by a Daniell cell, a sensitive galvanometer, and
some aluminium filings in an ebonite cup. The galvanometer needle
remained at zero. The filings were cut out of this circuit, and
switched for one minute into circuit with a battery of 43 sulphate of
mercury cells. On being replaced in the first circuit, the filings
exhibited high conductivity. The result was the same when 10 or 20
cells were employed, or when the current was diminished by interposing
in the circuit a column of distilled water, 40 cm. long and 20 mm.
in diameter. The cells used (platinum, sulphate of mercury, sulphate
of zinc, zinc) had a high internal resistance. Thus, 43 cells (60
volts), when short-circuited, only gave a current of 5 milliamperes.
The same battery, with the column of distilled water in circuit only,
caused a deflection of 100 mm. on a scale one metre off, with an
astatic galvanometer wound with 50,000 turns. We can, therefore, see
how infinitesimally small the initial current must have been when
the filings were added to the circuit. The battery acted, therefore,
essentially by virtue of its electromotive force.
If mechanical displacement of particles or transportation of conducting
bodies seem inadmissible, it is probable that there is a modification
of the insulator itself, the modification persisting for some time
by virtue of a sort of “coercive force.” An electric current of high
potential, which would be completely stopped by a thick insulating
sheet, may be supposed to gradually traverse the very thin dielectric
layers between the conducting particles, the passage being effected
very rapidly if the electric pressure is great, and more slowly if the
pressure is less.
_Increase of Resistance._--An increase of resistance was observed
in these investigations less often than a diminution; nevertheless,
a number of frequently repeated experiments enable me to say that
increase of resistance is not exceptional, and that the conditions
under which it takes place are well defined. Short columns of antimony
or aluminium powder when subjected to a pressure of about 1 kilogramme
per square centimetre (14·2 lb. per square inch), and offering but
a low resistance, exhibited an increase of resistance under the
influence of a powerful electrification. Peroxide of lead, a fairly
good conductor, always exhibited an increase; so also did some kinds of
platinised glass, while others showed alternate effects. For instance,
a sheet of platinised glass, which offered a resistance of 700 ohms,
became highly conducting after 150 sulphate of mercury cells had been
applied to it for 10sec. This condition of conductivity was annulled by
contact with a charged Leyden jar, and reappeared after again applying
150 cells for 10sec., and so on. Similar effects were obtained with a
thin layer of a mixture of selenium and tellurium poured, when fused,
into a groove in a sheet of mica placed between two copper plates.
These alternations were always observed several times in succession,
and at intervals of several days.
These augmentations and alternations are in no way incompatible
with the hypothesis of a physical modification of the insulator by
electrical influence.