_The following, written by Dr. Oliver Lodge, appeared in_ “THE
ELECTRICIAN” _for November 12th, 1897_:--
Probably the earliest discovery of cohesion under electric influence
was contained in that old, forgotten observation of Guitard in 1850,
that when dusty air was electrified from a point the dust particles
tended to cohere into strings or flakes. The same thing no doubt occurs
in the formation of snowflakes under the influence of atmospheric
electrification; and the cohesion of small drops into large ones in
the proximity of a charged cloud is exceedingly familiar, since it
results in the ordinary thunder-shower. Great light was thrown on
these meteorological phenomena by the discovery of Lord Rayleigh
in 1879 of the curious behaviour of a small fountain or vertical
water-jet when exposed to the neighbourhood of a stick of excited
sealing-wax. A smooth orifice being arranged to throw a jet of water
about three or four feet nearly vertically, the jet breaks into drops,
and the drops scatter in all directions, rebounding from one another
and giving a shower of fine spray; but if a stick of sealing-wax be
rubbed on the sleeve of a coat and brought within one or two yards of
the place where the jet breaks into drops, it will be found that the
scattering ceases, the fine spray is no longer formed, and the broken
jet rises and descends in great blobs of water. The rain-shower has,
in fact, been converted into a thunder-shower. Further experiments,
conducted chiefly with two jets, elucidated the phenomenon.[18]
Arranging two nearly parallel jets from neighbouring orifices so as
to impinge against each other, they were found ordinarily to rebound
after colliding, a sort of film or superficial layer appearing to
prevent amalgamation of the jets into one; but if a slight difference
of electric potential were maintained between the two jets, say by
connecting them to the terminals of a Leclanché cell, then the boundary
layer broke down,--the two colliding jets no longer separated with a
rebound, but amalgamated and became one.
[18] _Proc._ Roy. Soc., 1879 and 1882.
Lord Rayleigh developed a similar explanation for the single jet. The
scattering of the jet in its ordinary state was due to the rebound of
colliding drops, as could be seen by examining it with a sufficiently
instantaneous or intermittent mode of illumination; but if an electric
charge were in the neighbourhood it must be supposed that a trace of
potential difference existed between the drops, which caused them to
amalgamate into one whenever they collided, and thus speedily to become
united into a comparatively few large drops, which then continued on
their parabolic way.
At first sight it would seem as if the neighbourhood of a negative
charge should charge all the drops positively at the place whence they
break off from the earth-connected parent jet, and should thus cause
them all to repel each other. And if the electrified sealing-wax is
held too close, this is exactly what happens. All the drops are then
similarly electrified, and scatter more violently than ever, never in
that case coming into any rebounding or other contact with each other.
But under a gentler electric influence the similar charging has a less
marked result, and a polarisation difference of potential of one or two
volts may without difficulty be supposed to exist in the air between
drops, partly because they are not all equally charged and partly
because each is a conductor acted on inductively by a neighbouring
electrified body. In this connection it must be remembered that rubbed
sealing wax is at a potential of several thousand volts, and therefore
can readily cause a potential gradient of two or three volts per
millimetre throughout a yard or two of space.
The next stage was the re-discovery, in 1883, of Guitard’s old dust
phenomenon by the present writer and the late J. W. Clark (_Nature_,
July, 1883; _Phil. Mag._, March, 1884), when they were working together
at the dust-free region seen over hot bodies when strongly illuminated
in dusty air. The fact of such dust-free spaces was discovered by
Tyndall, and they can readily be seen by placing a lighted spirit lamp
or a hot poker in the beam of an electric lamp. Tyndall thought the
dust was calcined or burnt up, and that thus the air was freed from it;
but this is an utterly erroneous explanation, and the true explanation
is of a more recondite character, being connected with the bombarding
effect of gas molecules as illustrated in the Crookes radiometer.
The dust particles are beaten away from the hot body by a molecular
bombardment, which manifests itself even at ordinary pressures on
bodies of sufficiently small size, as indeed was also otherwise shown
by Tait and Dewar and Osborne Reynolds in the course of remarkable
theoretical and practical investigations.[19]
Before arriving at this explanation, however, we experimented to
see if the phenomenon was caused by the air having become slightly
electrified, perhaps by reason of its having streamed as an upward
convection current over the surface of the warm solid, at which we were
looking, in a thick smoky atmosphere, in the concentrated light of an
electric arc. We therefore purposely electrified the rod, to see what
that would do, and we found to our surprise, directly the electric
machine was turned, that the smoky atmosphere almost instantaneously
disappeared, and the box became quite clear.
This experiment, after development, though described in July, 1883,
was shown in public for the first time at the Dublin Royal Society
(_Nature_, April 24, 1884), and subsequently at the British Association
meeting in Montreal[20] in 1884, and was applied to the experimental
clearing of rooms from dense smoke or fume. It has often been shown
since, by Mr. Swan and others, and has become fairly well known.[21]
[19] “Dimensional Properties of Matter,” _Phil. Trans._, 1879.
[20] Evening Lecture on “Dust,” by the writer, see _Nature_, Vol. 31,
p. 265; also _Journal_ of the Royal Institution, May, 1886.
[21] Apparatus for the purpose is now in the catalogue of Messrs.
Ducretet, of Paris, but they supply a pair of combs of points. It
makes a more interesting experiment if only one point is used, in a
moderate space, and the electric supply regulated so as not to hurry
the disappearance of the smoke too quickly, but to exhibit the stages
of aggregation which precede the final disappearance by deposition. Any
kind of smoke serves, but a bit of magnesium ribbon burnt under a bell
jar is cleanly and effective. It should be looked at in a window or
other good light, of course.
The next observation of cohesion under electrical influence was made
by the writer in 1889, while working at the protection of telegraphic
instruments and cables from lightning,--a research which resulted in
the use of choke coils as supplementary to the air gaps of the ordinary
lightning guard, and thus to the forms of instrument constructed by
Dr. Alex. Muirhead for telegraphic work in this country, and to the
supplementary additions adopted by the Westinghouse Company for their
non-arcing guards adapted to electric light and power installations in
America. The observation of cohesion was a bye-issue, noticed when the
knobs of the lightning guard were brought too close together.[22]
[22] _Journal_ of the Institution of Electrical Engineers for 1890, pp.
352-4.
When lightning itself strikes a guard, it has indeed often been found
that the opposite sides of the protective air gap are fused together.
This, no doubt, may be partly due to a straightforward melting or
welding by heat, but it is probably not solely that. Molten metals
without a flux do not so readily weld. It is almost certainly due to a
cohesive action also, the difference of potential between the molten
terminals resulting in adhesion and amalgamation, a phenomenon also
observed in the frequent locking of an electric arc formed between two
metallic electrodes. However this may be, certainly the phenomenon
occurs on a small scale, for if the pair of knobs or points placed
as a shunt to protect a galvanometer or other telegraphic instrument
from lightning (or what is easier experimentally and essentially the
same thing, from a Leyden jar discharge) be set too close together,
the galvanometer will be found to be short-circuited after a spark,
and the knobs will be found, both by mechanical and electrical tests,
to be feebly united at a single point.[23] Not only, however, is the
galvanometer short-circuited by the metallic junction so formed, but
at the instant of the formation of the joint it experiences a very
perceptible kick, indicating a momentary current, coincident no doubt
with the electric discharge, but one from which it would have been
protected had not the junction occurred. The galvanometer kick is
clearly an effect due to the uniting metals, but it has not yet been
fully elucidated; it seems to have been first observed by Mr. Stroh
in his excellent researches on microphonic action, related in the
_Journal_ of the Society of Telegraph Engineers, 1883 and 1887, and it
may possibly be thermo-electric, as Prof. Hughes, who also observed it,
thinks likely; but it may be electro-chemical, or it may be connected
with an effect observed later by FitzGerald in his galvanometer mode of
detecting Hertzian waves, which he published at the Royal Institution
in 1890. The point of present interest is the cohesion which sets in
between the knobs when the spark occurs: an extremely feeble spark was
found sufficient to produce the effect, provided the surfaces were
already almost infinitely close together, _i.e._, provided they were
already in what would be called contact, with the merest imperceptible
film of (probably) oxide separating them, just the kind of film
which a chemical flux is useful in removing. The electrical stimulus
appears to act as such a flux, and the adhesion of the two surfaces
was demonstrated by an electric bell and single cell in circuit. Every
time the spark occurred the bell rang, and continued ringing, until
the table, or some part of the support of the knobs, was tapped so as
to shake or jar them asunder again.[24] The arrangement constitutes a
convenient detector in the syntonic Leyden jar experiment, depicted in
Fig. 4, p. 6 (_see_ also p. 21).
[23] “Modern Views,” second edition, p. 359.
[24] _Journal_ of the Institution of Electrical Engineers, 1890, p.
352. _See_ also remarks by Mr. Stroh in two microphone discussions,
_Journal_ of the Institution of Electrical Engineers, 1883 and 1887.
If the electric bell stands on the same table as the support of the
sparking knobs, or, still better, if it be put into mechanical contact
with them, its tremor is quite sufficient to break the contact asunder
again; unless the spark, and therefore the adhesion, has been too
strong. Raising the bell into the air, it ceases to interrupt the
spark-induced continuity, and in that case continues to ring; but
directly it is replaced so that its vibration can reach the cohered
surfaces through their solid supports it usually happens that a few
strokes--often, indeed, the first stroke--of the bell, sometimes even
the incipient movement of the hammer preparatory to a stroke, is
sufficient to break the circuit and suspend instantly the action,
restoring the gap to its original condition and leaving the circuit
ready to be completed again by another spark.
The spark in these early experiments was usually supplied from the
outer coats of a pair of oppositely-charged small Leyden jars, whose
knobs sparked into each other; the idea being to ascertain all the
conditions pertaining to the feeble residue of a lighting discharge
which is liable to be conducted by telegraph wires to a distance, and
there cause some damage to sensitive instruments not suitably protected
from sudden electric jerks, whose laws of flow are quite different from
those proper to steady currents.
Meanwhile, in 1887 and 1888, had been performed the great experiments
of Hertz on electric waves in free space. The writer, assisted by Prof.
Chattock, had also made some experiments concerning the production
and detection of waves on a system of long parallel wires stretched
on insulators across and around a large room, and excited by the
discharge of a pair of condensers, an arrangement very similar to that
now known under the name of Lecher; and clear experimental evidence of
the existence of nodes and loops on such wires, as well as a method
of approximately measuring the wave length, was given.[25] The brush
luminosity of the wires, afterwards observed more strikingly by Tesla,
was also seen and shown to the Physical Society. The interest of
these experiments was, however, altogether eclipsed by the brilliant
and masterly investigation at Carlsruhe by Hertz, who, as everyone
except the British public is aware, put into practice FitzGerald’s
1883 suggestion that Leyden jar discharges should emit Maxwellian
radiation, and conclusively demonstrated the existence and some of the
properties of such waves by this very means; using, however, Leydens
of small capacity, and with the coatings well separated, so that the
electrostatic energy of the charge should have an intensity comparable
with the magnetic energy of the discharge, even at some distance from
the circuit.
[25] Verbally to Section A at Bath, 1888. _See_ also _Phil. Mag._,
August, 1888, p. 229; and _The Electrician_, Vol. 21, pp. 607-8.
The whole subject of electric waves was thus laid open to physicists,
and many have been the workers in the field. Trouton, of Dublin,
worked long and successfully at their optical analogies, with the
very inadequate means of detection then known;[26] and since better
means have been known perhaps the most complete set of experiments
published, after Hertz himself, is that contained in the book “Optice
Elettrica,” by Prof. Righi, of Bologna; but some account of several
previous researches is contained in the second edition of “Modern Views
of Electricity,” in the chapter called “Recent Progress,” of date 1892.
The means used by Hertz and his immediate followers to detect the waves
was simply the little spark which they excited in conductors upon which
they fell; electric currents being set up in such conductors by the act
of reflection. The effect was often at that time attributed to electric
resonance or syntony, but there was very little true resonance in these
experiments; the first swing was usually much more powerful than any of
the succeeding ones, and was competent to cause the little spark; if it
failed the remainder of the swings had but a poor chance of success.
Consequently precision of tuning was not really important, though no
doubt it would help a little.
[26] _Nature_, Vols. 39 and 40.
It is interesting to note that a magnetic needle detector not unlike
Rutherford’s had been used long ago by Joseph Henry at Washington,
and that minute induced sparks, identical in all respects with those
discovered by Hertz, had been seen in recent times both by Edison and
by Silvanus Thompson, being styled “etheric force” by the former; but
their theoretic significance had not been perceived, and they were
somewhat sceptically regarded. Yet Henry, even in those pre-Maxwellian
days, was led to an intuition concerning the spread of electrical
disturbance surprisingly near the truth. The truth indeed it was in
some sort, but it was not worked out or grasped in detail, and so
cannot be considered as more than a brilliant guess; but the fact that
an observation of the widespread surgings induced in the neighbourhood
of a primary discharge had been made by Henry, and had been seen by
others to be capable of giving actual sparks, before the time of Hertz,
although it has no real bearing on Hertz’s fresh discovery, and did
not lead those who, like the writer, had long been trying to think
of a detector for Maxwellian waves to discover one, nevertheless is
instructive as showing how frequently it happens that a fact is lying
ready to hand but is not taken up and appreciated until some special or
extra stimulus has been supplied.
After Hertz’s results had become well-known, the writer devised a plan
whereby real electric resonance could be demonstrated with a pair of
actual glass Leyden jars of ordinary pattern, by connecting each to
a discharge circuit, the one complete, the other with an air gap,
and providing the first or receiving jar with an overflow path or
bye-circuit provided with an air gap across which a visible spark could
occur whenever the induced oscillations or surgings accumulated in its
main circuit were sufficiently intense to make the jar overflow.[27]
The air gap was most easily provided by a strip of tinfoil pasted
over the lip of the jar, but it served equally well if wires led from
the two coatings to a pair of adjustable knobs near together, like
a lightning guard, between which the overflow spark could pass. The
same knobs indeed were used as had already served for the lightning
experiments; and, as in that case, if the knobs are arranged very
close together and are put in circuit with a battery and a bell,
cohesion sets in and the bell rings whenever the overflow occurs. The
bell continues to ring until the stand is tapped, but if the bell
itself touches the stand or the table, it rapidly breaks contact by
its vibration, exactly as described, p. 77 (_see_ also Fig. 16A, p.
21). Closed Leyden jar circuits are not strong radiators, nor was this
resonance then observed excited by true waves. No attempt was at this
time made to apply the cohesion principle to the detection of true
Hertz waves such as could be felt at a considerable distance from a
strongly radiating source.
Before this time, FitzGerald and Trouton had hit upon their
galvanometer method of demonstrating to an audience the occurrence of
the minute scarcely-visible spark in the gap of a Hertz receiver.[28]
[27] _Nature_, Vol. 41, p. 368; or, “Modern Views of Electricity,”
second edition, p. 338. _See_ also Fig. 4, p. 6.
[28] _Nature_, Vol. 41, p. 295; and Vol. 42, p. 172.
Prof. Minchin also, working at Cooper’s Hill with his sensitive
photo-electric cells, especially with some which he called “impulsion
cells,” that behaved abnormally when subjected to taps or other
mechanical vibrations, found that when Mr. Gregory was working a Hertz
radiator in another part of the same laboratory the electrometer
connected to his cells responded.[29] Many other detectors have been
devised and used, but this of Minchin’s almost certainly depends on the
cohesion principle, though its action seemed paradoxical then. Moreover
he was able, by its aid, to signal without wires over a considerable
number of yards, at that early date (1890 and 1891).
About the same time, Prof. Boltzmann used a charged gold-leaf
electroscope for the same purpose, having it so arranged that the
electroscope was on the point of discharging across a minute air gap,
so that its leaves were dilated by a definite amount. The slightest
excess of charge would make it discharge and the leaves instantly
collapse. In this charged condition it was sensitive to very minute
electric surgings, and if Hertz waves were excited in another part of
the room, the wave disturbances caused the gap to break down and the
electroscope leaves to collapse.[30] This method is not a cohesion
method, but it led the writer, when subsequently repeating Boltzmann’s
results with modifications, to realise that, if the gap were almost
closed, cohesion could be made to set in by the surgings induced by
regular Hertz waves (Fig. 16, p. 18).
[29] _Phil. Mag._, March, 1891; also January, 1894.
[30] _Wied. Ann._, Vol. 40.
The Boltzmann gap method was accordingly modified in several ways;
one way was to make it of carbon and to connect it, with its wave
collector, to the terminals of 110-volt electric light leads, so that
whenever a Hertz vibrator was discharged and induced a minute spark
across the gap, that same spark might close the circuit and establish
an arc. This plan forced itself on my attention by the behaviour of
sundry Swan lamps suspended with shades so as to illuminate my lecture
table, which became short-circuited whenever a large Hertz vibrator
was at work; for the lamps were at that time kept from rotation, and
thereby from glaring into the eyes of the audience instead of being
screened from them, by a couple of copper wires stretched across the
theatre. So long as those wires were there, the fuses used to blow
whenever a Hertz oscillator was started; an experiment which was
interesting enough, and was shown to several people, including, I
think, Prof. FitzGerald, but which was sufficiently a nuisance to
necessitate the wires, which were acting as collecting wires, being
taken down and replaced by stretched silk threads, which are there to
this day. Another modification was to connect the gap to an Abel’s
fuse or to a gas leak, which exploded or ignited under the influence
of a feeble spark. Yet another was to connect it to a single cell and
electric bell or galvanometer, as already explained.
Meanwhile, however, and well before these later experiments on the
detection of Hertz waves were in progress, certain discoveries had been
made by M. Branly, Professor of Physics in the Catholic Institute of
Paris, which were of the greatest interest and importance. Prof. Branly
had found that a coat or varnish of fine copper dust, porphyrised
copper or other such substance, though it could only conduct a current
very feebly, and much as a blacklead pencil trace conducts, under
ordinary conditions, yet fell in resistance enormously whenever an
electric spark occurred in its neighbourhood; somewhat in the fashion
that the resistance of selenium falls on exposure to light. It is not
clear that M. Branly recognised that he was dealing with Hertz waves or
true electrical radiation, but his observations were most satisfactory
and conclusive, and he measured the reduction of resistance caused in
a number of different substances, including an assemblage of metallic
filings, and conglomerates or paste of filings in various viscous
liquids and in dry powders. Moreover, he found that the spark was still
operative in reducing resistance even when it was several yards distant.
The account of Prof. Branly’s experiments is to be found in a couple
of short communications to the French Academy of Science (_Comptes
Rendus_, Vols. 111 and 112), and the writer had intended to reproduce
in abstract the gist of these memoirs; but to readers of _The
Electrician_ this is unnecessary, as a descriptive article from _La
Lumière Electrique_ has already been translated in full, in July and
August, 1891 (see _The Electrician_, Vol. XXVII., pp. 221 and 448, now
reproduced as Appendix). Unfortunately the writer, in common perhaps
with others, must confess to having overlooked these articles at the
time, probably by reason of their coincidence with the holiday season.
In his second edition of “Modern Views of Electricity,” published in
1892, though he refers on page 359 to the cohesion principle in this
connection, the writer is clearly ignorant of Branly’s experiments.
The matter seems to have been ignored in this country till 1892, when
Dr. Dawson Turner described the experiments to the British Association
in Edinburgh, and even till 1893, when Mr. Croft brought them to the
notice of the London Physical Society. Prof. Minchin at once realised
that here was a phenomenon analogous to what he had been observing
with his impulsion cells, and after a few trials wrote a Paper to
the Physical Society recounting his repetitions and modifications
of Branly’s experiments.[31] This Paper, before it was read, was
circulated by the Society to its country members, and so came to the
eye of the writer, who at once wrote a short note summarising some of
his work in the same direction, and pointing out that this discovery of
Branly’s, thus made known to him, was another case of the electrical
cohesion phenomenon already observed by several experimenters. This
is published along with Prof. Minchin’s Paper in the _Phil. Mag._ for
January, 1894, and to it the friendly reader is referred. The writer
at once proceeded to try the Branly tube of filings, and found it
far superior in manageability to either the Boltzmann gap or his own
delicately adjusted cohering knobs; though immediately afterwards
he and FitzGerald together arranged a single-point coherer, of iron
and aluminium (point of sewing needle resting on aluminium foil), of
what was at that time extraordinary sensitiveness and of reasonable
manageability. A whole series of quasi-optical experiments were then
undertaken with the new detector, and were shown to students and to the
Liverpool Physical Society; moreover, before long, various improved
methods of arranging the filings were gradually adopted, especially
by sealing them up in vacuum or in an atmosphere of hydrogen (_see_
page 34) so as to protect them from continued oxidation by the air,
and to prevent the film which hypothetically separates the surfaces
from growing too thick. Indeed, brass filings in hydrogen speedily got
_too_ clean, and became so sensitive that it was almost impossible
to restore the original high resistance by tapping. Consequently,
a perfect or Sprengel vacuum was preferred to hydrogen. Almost any
filings tube could detect signals from a distance of 60 yards, with
a mere six-inch sphere as emitter and without the slightest trouble,
but the single-point coherer was usually much more sensitive than any
filings tube. Mr. Shelford Bidwell has also worked with varieties of
powder.
[31] _Phil. Mag._, January, 1894.
The tapping back was at first performed by hand, and for optical
experiments this is still, perhaps, the most convenient plan; but
automatic tappers were very soon arranged, just as with the old
knobs; an electric bell mounted on the base of a filings tube (_see_
page 31) was not found very satisfactory, however, because of the
disturbances caused by the little sparks at its contact-breaker, to
which the previous coarser knob-arrangements had failed to respond; so
a clockwork tapper, consisting of a rotating spoke wheel driven by the
clockwork of a Morse instrument, and giving to the filings tube or to
a coherer a series of jerks occurring at regular intervals, to imitate
what the writer supposed must occur in the eye, viz., a restoration to
sensitiveness after an interval corresponding to the persistence of
impression, was also employed. Many of these things were shown at a
Friday evening lecture at the Royal Institution on June 1, 1894, while
others were shown the same autumn at the B.A. meeting at Oxford. In
both cases signalling was easily carried on from a distance through
walls and other obstacles, an emitter being outside and a galvanometer
detector inside the room. Distance without obstacle was no difficulty
in these experiments, only free distance is not very easy to get in
a town, and stupidly enough no attempt was made to apply any but the
feeblest power so as to test how far the disturbance could really be
detected. Mr. Rutherford, however, with a magnetic detector of his own
invention, constructed on a totally different principle, and probably
much less sensitive than a coherer, did make the attempt and succeeded
in signalling across half-a-mile, full of intervening streets and
houses at Cambridge.[32]
[32] _Phil. Trans._, 1897, A., communicated to the Royal Soc., June,
1896.
Numbers of people have worked at the detection of Hertz waves with
filing tube receivers, and every one of them must have known that
the transmission of telegraphic messages in this way over moderate
distances was but a matter of demand and supply; Sir W. Crookes,
indeed, had already clearly stated this telegraphic application of
Hertz waves in the _Fortnightly Review_ for February, 1892, and refers
to certain experiments already conducted in that direction,[33] the
details of which are unknown to the writer (but see Appendix I.).
There remained no doubt a number of points of detail, and considerable
improvements in construction, if the method was ever to become
practically useful; but these details could safely be left to those
who had charge of the Government monopoly of telegraphs, especially as
their eminent Head was known to be interested in this kind of subject.
[33] Quoted in _The Electrician_ “Notes,” October 1, 1897.
Meanwhile the optical developments of the matter excited most interest
among physicists, both here and on the continent; the writer performed
some experiments of the kind, Prof. Righi at Bologna performed many
more, and Prof. Chunder Bose, of Calcutta, repeated several of them
with additions and improvements, using as detector a sort of half-way
house between a point coherer and a filings tube by squeezing a few
rolls or spirals of wire between a point and a micrometer screw.
Restoration to sensitiveness was in this case achieved by relaxing
the pressure of the screw, and the writer has not found Bose’s form
of coherer specially convenient; but Prof. Bose’s whole apparatus,
constructed as it was precisely on lines published by the writer, was
well designed in detail and exceedingly compact, being on the scale
of an ordinary goniometer; and with it many experiments familiar in
ordinary optics could readily be shown with electric radiation.
In all the optical experiments made by any of these observers it was
customary to place the axis of the emitter either horizontally, or
vertically, or inclined, in other words to emit radiation polarised in
any azimuth (or rather altitude), and to arrange the collecting part of
the receiver to correspond or otherwise, according as response or no
response was desired. In fact, observations on polarisation were the
easiest and the most instructive that could be made with the definite
kind of radiation now for the first time at command. The rotation of
the plane of polarisation, the conversion of plane into elliptical
polarisation, the amount of radiation reflected by substances at
different angles and different aspects with regard to the direction
of vibration, were readily observed. Furthermore, ever since Hertz’s
first discovery, whenever waves had to travel through a metal grid or
alongside a plane conductor, it was natural to arrange the electric
oscillations so as to be normal to the conducting lines or plane, for
if they were tangential they excited electric currents therein, and
their energy became wasted in the production of heat. So, in so far
as earth and water are conductors, it is desirable to use radiation
polarised in a horizontal plane, _i.e._, with the electric oscillations
vertical, if considerable distances are to be traversed by it.
With respect to an explanation _why_ metallic cohesion is caused under
electrical influence, the following considerations are offered:--
Mr. Rollo Appleyard made a liquid coherer of two globules or pools of
mercury, side by side and touching, but kept apart by a thin film of
grease, such as is easily given by a coat of paraffin oil. Connecting
up a battery cell to these mercury pools through a key, he found that
every time the key is depressed the pools move together and become
one; he points out moreover that mercury globules shoot out a tentacle
towards the positive terminal (on the principle of the capillary
electrometer, of course), and this must be taken into account in
any coherer theory.[34] Lord Rayleigh also devised and exhibited a
liquid form of coherer. It is interesting to observe, as he points
out, that in a mercury form of coherer an appreciable time interval
occurs between the depression of the key and the amalgamation of the
mercury, the lag looking as if a film had to be mechanically squeezed
out between the oppositely-charged mercury surfaces, and as if this
took a perceptible fraction of a second to accomplish. This experiment
conveys the useful suggestion that cohesion may in all cases be the
result of electrostatic attraction, and that the molecular films
separating solids in contact may thus also have to be squeezed out,
though as they only touch at single points such extrusion is almost
instantaneously achieved. This may very likely be the chief cause, for
although a true electro-chemical extension of the range of cohesion
between polarised molecules had seemed to the writer to be a possible
explanation also, he now perceives that the electrostatic force alone
may be sufficient. For it is easy to calculate the force of attraction
between two surfaces differing in potential by a volt, and separated
from one another by the smallest known thickness of thin film (which
is 10⁻⁷ centimetre, or 1 millimicron, called μ μ by microscopists);
such force per unit area would be given by the square of the potential
gradient divided by 8π, that is, it would amount to
┌ ┐2
1 │ 10⁷│
----│----│ dynes per square centimetre,
25 │ 300│
└ ┘
which equals 44 atmospheres, and is a very considerable pressure. A
hundred times this attractive pressure would exist if the surfaces
were within really _molecular_ distance of each other; in addition to
the force of true cohesion which would then, still more powerfully,
operate; but the film thickness assumed above is such as would just
prevent the force of cohesion from effectively acting across the
gap, and would leave the electrical attraction due to the one volt
alone. Three and a half volts could therefore squeeze metals together
with a force equal to a ton load per square inch, and might thus be
sufficient to cause them to weld or unite, especially if the electric
stimulus simultaneously acted in any way as a flux, by reducing the
infinitesimal tarnish of oxide or other compound which must be supposed
normally to cover them.
[34] _Phil. Mag._, May and July, 1897. He also shows electrical
cohesion by an emulsion of oil and water, the two liquids, thoroughly
shaken up, at once separating when exposed to strong electrical
influence.
In so far as the approximate contact is not between _surfaces_, but
between points consisting of relatively few molecules, the attractive
pressure is greater rather than less. Thus to take an extreme case,
the attraction between two oppositely-charged molecules differing
only by a volt from each other, and separated by a thin film like the
black spot of a soap-film whose thickness was so admirably measured
by Profs. Reinold and Rücker, is over 1,000 atmospheres in intensity.
These differences of potential across thin films cannot continue
for any time, unless a battery is used, for the films do not really
insulate; they are able however to act as dielectrics for an instant,
and to be burst with what we must be allowed to call a spark, though
an infinitesimally small one, if the momentary strain caused by the
impulsive rush of electricity is too great.