Chapter III: Part 3

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.

APPENDIX I.

PROF. HUGHES’S OBSERVATIONS.

An account of the history of the coherer principle would not be complete without a reference to an interesting reminiscence of early observations recently put on record by the discoverer of the microphone. At each stage of his observations of electrical cohesion between metals the author was confronted by a reference to some earlier observations of Prof. Hughes, and he felt sure that during the work on the microphone many or all of the phenomena he was then observing must have been previously encountered by Prof. Hughes. No full account was at that time available, however, but now it is clear that the observations were made (like some of Edison’s on what he called etheric force, and like the very remarkable still earlier ones of Joseph Henry) before the time of Hertz, when the existence of electric waves able to excite sparks or perform other energetic acts was unlooked for and not clearly understood.

Nevertheless, at this early period it is clear that Prof. Hughes observed, though he did not follow up the observation, not only the occurrence of electric waves or impulses in space, but also the coherer method of detecting them; in fact, that he unwittingly made the earliest experiments on wireless telegraphy by this plan.

The simplest way is to quote Prof. Hughes’s letter to Mr. J. J. Fahie from _The Electrician_, May 5, 1899, p. 40, beginning with Mr. Fahie’s letter as an introduction:--

_Extract from recent letter from Mr. J. J. Fahie to
Prof. Hughes._

“DEAR PROF. HUGHES: I have now in the press
a history of Wireless Telegraphy from 1838 to 1899,
and in writing to Sir William Crookes for information
he tells me that many years ago he saw some
experiments of yours with the microphone, in which
you signalled from one part of a house to another
without connecting wires, and he desires me to refer
to you for particulars. I think, with Sir William,
that it is a pity you have not hitherto published
your results, and I sincerely hope you will now do
so. If also you would kindly favour me with a short
account, I could find room for it in my book, which
is now in the printer’s hands.--Sincerely yours,

J. J. FAHIE.

Claremont Hill, St. Helier’s, Jersey,
April 26, 1899.”

_Reply from Prof. D. E. Hughes_:--

40, Langham-street, W., April 29, 1899.

DEAR SIR: In reply to yours of the 26th
inst., in which you say that Sir William Crookes has
told you “that he saw some experiments of mine on
aërial telegraphy, in about December, 1879, of which
he thinks I ought to have published an account,” and
of which you ask for some information, I beg to reply
with a few leading experiments that I made on this
subject from 1879 up to 1886:--

“In 1879, being engaged upon experiments with my
microphone, together with my induction balance, I
remarked that at some time I could not get a perfect
balance in the induction balance, through apparent
want of insulation in the coils, but investigation
showed me that the real cause was some loose contact
or microphonic joint excited in some portion of
the circuit. I then applied the microphone in
the circuit, and found that it gave a current or
sound in the telephone receiver, no matter if the
microphone was placed direct in the circuit, or
placed independently at several feet distance from
the coils, through which an intermittent current was
passing. After numerous experiments, I found that
the effect was entirely caused by the extra current
produced in the primary coil of the induction balance.

“Further researches proved that an interrupted
current, in any coil, through which an electric
current was sent, gave out at each interruption of
the primary current, such intense extra currents,
that the whole atmosphere in the room (or in several
rooms distant) would have a momentary invisible
charge, which became evident if a microphonic joint
was used as a receiver to a telephone. This led me
to experiment upon the best form of a receiver for
these invisible electric waves, which evidently
permeated great distances, and through all apparent
obstacles, such as walls, &c. I found that all
microphonic contacts or joints were extremely
sensitive. Those formed of a hard carbon such as
coke, or a combination of a piece of coke resting
upon a bright steel contact, were very sensitive and
self-restoring; whilst a loose contact between metals
was equally sensitive, but would cohere, or remain in
full contact, after the passage of an electric wave.

“The sensitiveness of these microphonic contacts
in metals has since been rediscovered by Mons. Ed.
Branly, of Paris, and by Prof. Oliver Lodge, in
England, by whom the name of ‘coherer’ has been given
to this organ of reception; but, as we wish this
organ to make a momentary contact and not cohere
permanently, the name seems to me ill-suited for the
instrument. The most sensitive and perfect receiver
that I have yet made does not cohere permanently,
but recovers its original state instantly, and,
therefore, requires no tapping or mechanical aid to
the separation of the contacts after momentarily
being brought into close union.

“I soon found that, whilst an invisible spark would
produce a thermo-electric current in the microphonic
contacts (sufficient to be heard in the telephone in
its circuit), it was far better and more powerful to
use a feeble voltaic cell in the receiving circuit,
the microphonic joint then acting as a relay, by
increasing and diminishing the resistance at the
contact, by the influence of the electric wave
received through the atmosphere.

“I will not describe the numerous forms of the
transmitter, and receiver, that I made in 1879,
all of which I wrote down in several volumes of
manuscripts in 1879 (but these have never been
published), most of which can be seen here at my
residence at any time; but I will confine myself
now to a few salient points. I found that very
sudden electric impulses, whether given out to the
atmosphere through the extra current from a coil, or
from a frictional electric machine, equally affected
the microphonic joint, the effect depending more on
the sudden high potential effect than any prolonged
action. Thus, a spark obtained by rubbing a piece of
sealing-wax was equally as effective as a discharge
from a Leyden jar battery of the same potential. The
rubbed sealing-wax, or charged Leyden jar, had no
effect, until they were discharged by a spark,--and
it was evident that this spark, however feeble, acted
upon the whole surrounding atmosphere in the form of
waves, or invisible rays, of which I could not at
the time determine. Hertz, however, by a series of
original and masterly experiments, proved in 1887-9,
that they were real waves similar to light, but of
a lower frequency, though of the same velocity. In
1879, whilst making these experiments on aërial
transmission, I had two different problems to solve:
1st, What was the true nature of these electric
aërial waves, which seemed, whilst not visible, to
spurn all idea of insulation, and to permeate all
space to a distance undetermined. 2nd, To discover
the best receiver that could act upon a telephone or
telegraph instrument, so as to be able to utilise
(when required) these waves for the transmission of
messages. The second problem came easy to me, when
I found that the microphone, which I had previously
discovered in 1877-8, had alone the power of
rendering these invisible waves evident, either in
a telephone or galvanometer, and up to the present
time I do not know of anything approaching the
sensitiveness of a microphonic joint as a receiver.
Branly’s tube, now used by Marconi, was described in
my first Paper to the Royal Society (May 8, 1878),
as the microphone tube, filled with loose filings
of zinc and silver, and Prof. Lodge’s coherer is
an ordinary steel microphone, used for a different
purpose from that in which I first described it.

“During the long-continued experiments on this
subject, between 1879 and 1886, many curious
phenomena came out which would be too long to
describe. I found that the effect of the extra
current in a coil was not increased by having an
iron core as an electromagnet--the extra current
was less rapid and, therefore, less effective. A
similar effect of a delay was produced by Leyden jar
discharges. The material of the contact-breaker of
the primary current had also a great effect. Thus,
if the current was broken between two, or one, piece
of carbon, no effect could be perceived of aërial
waves, even at short distances of a few feet. The
extra current from a small coil, without iron, was as
powerful as an intense spark from a secondary coil,
and at that time my experiments seemed to be confined
to the use of a single coil of my induction balance,
charged by six Daniell cells. With higher battery
power, the extra current invariably destroyed the
insulation of the coils.

“In December, 1879, I invited several persons to see
the results then obtained. Amongst others who called
on me and saw my results were:--

“December, 1879.--Mr. W. H. Preece, F.R.S.; Sir
William Crookes, F.R.S.; Sir W. Robert Austen, F.R.S.;
Prof. W. Gryll Adams, F.R.S.; Mr. W. Groves.

“February 20, 1880.--Mr. Spottiswoode, Pres.R.S.;
Prof. Huxley, F.R.S.; Sir George Gabriel Stokes, F.R.S.

“November 7, 1888.--Prof. Dewar, F.R.S.; Mr. Lennox,
Royal Institution.

“They all saw experiments upon aërial transmission,
as already described, by means of the extra current
produced from a small coil and received upon a
semi-metallic microphone, the results being heard
upon a telephone in connection with the receiving
microphone. The transmitter and receiver were in
different rooms, about 60 ft. apart. After trying
successfully all distances allowed in my residence
in Portland-street, my usual method was to put the
transmitter in operation and walk up and down Great
Portland-street with the receiver in my hand, with
the telephone to the ear.

“The sounds seemed to slightly increase for a
distance of 60 yards, then gradually diminish, until
at 500 yards I could no longer with certainty hear
the transmitted signals. What struck me as remarkable
was that, opposite certain houses, I could hear
better, whilst at others the signals could hardly
be perceived. Hertz’s discovery of nodal points in
reflected waves (in 1887-9) has explained to me what
was then considered a mystery.

“At Mr. A. Stroh’s telegraph instrument manufactory,
Mr. Stroh and myself could hear perfectly the
currents transmitted from the third story to the
basement, but I could not detect clear signals at
my residence about a mile distant. The innumerable
gas and water pipes intervening seemed to absorb or
weaken too much the feeble transmitted extra currents
from a small coil.

“The President of the Royal Society, Mr.
Spottiswoode, together with the two hon. secretaries,
Prof. Huxley and Prof. G. Stokes, called upon me on
February 20, 1880, to see my experiments upon aërial
transmission of signals. The experiments shown were
most successful, and at first they seemed astonished
at the results, but towards the close of three hours’
experiments Prof. Stokes said, ‘that all the results
could be explained by known electromagnetic induction
effects, and therefore he could not accept my view of
actual aërial electric waves unknown up to that time,
but thought I had quite enough original matter to
form a Paper on the subject to be read at the Royal
Society.’

“I was so discouraged at being unable to convince
them of the truth of these aërial electric waves,
that I actually refused to write a Paper on the
subject, until I was better prepared to demonstrate
the existence of these waves; and I continued my
experiments for some years, in hopes of arriving at
a perfect scientific demonstration of the existence
of aërial electric waves, produced by a spark from
the extra currents in coils, or from frictional
electricity or secondary coils. The triumphant
demonstration of these waves was reserved to Prof.
Hertz, who by his masterly researches upon the
subject in 1887-9 completely demonstrating not only
their existence but their identity with ordinary
light, in having the power of being reflected and
refracted, &c., with nodal points, by means of which
the length of the waves could be measured, Hertz’s
experiments were far more conclusive than mine,
although he used a much less effective receiver than
the microphone or coherer.

“I then felt it was now too late to bring forward my
previous experiments, and through not publishing my
results, and means employed, I have been forced to
see others remake the discoveries I had previously
made as to the sensitiveness of the microphonic
contact, and its useful employment as a receiver for
electric aërial waves. Amongst the earliest workers
in the field of aërial transmission I would draw
attention to the experiments of Prof. Henry, who
describes in his work, published by the Smithsonian
Institute, Washington, D.C., U.S.A., Vol. I., p.
203 (date unknown, probably about 1850), that he
magnetised a needle in a coil at 30 ft. distance, and
magnetised a needle by a discharge of lightning at
eight miles distance.

“Marconi has lately demonstrated that by the use of
the Hertzian waves and Branly’s coherer he has been
enabled to transmit and receive aërial electric waves
to a greater distance than previously ever dreamed
of by the numerous discoverers and inventors who
have worked silently in this field. His efforts at
demonstration merit the success he has received;
and if (as I have lately read) he has discovered
the means of concentrating these waves on a single
point desired without diminishing its power, then
the world will be right in placing his name on the
highest pinnacle in relation to aërial electric
telegraphy.--Yours, &c.,
D. E. HUGHES.”

APPENDIX II.

VARIATIONS OF CONDUCTIVITY UNDER ELECTRICAL INFLUENCE.

The following is abstracted from an article by M. E. Branly in _La Lumière Electrique_ of May 16, 1891, and is taken from _The Electrician_ of June 26, 1891:--

The object of this article is to describe the first results obtained in an investigation of the variation of resistance of a large number of conductors under various electrical influences. The substances which up to the present have presented the greatest variations in conductivity are the powders or filings of metals. The enormous resistance offered by metal in a state of powder is well known; indeed, if we take a somewhat long column of very fine metallic powder the passage of the current is completely stopped. The increase in the electrical conductivity by pressure of powdered conducting substances is well known, and has had various practical applications. The variations of conductivity, however, which occur on subjecting conducting bodies to various electrical influences have not been previously investigated.

_The Effect of Electric Sparks._--Let us take a circuit comprising a single cell, a galvanometer, and some powdered metal enclosed in an ebonite tube of 1 square centimetre cross-section and a few centimetres long. Close the extremities of the tube with two cylindrical copper tubes pressing against the powdered metal and connected to the rest of the circuit. If the powder is sufficiently fine, even a very sensitive galvanometer does not show any evidence of a current passing. The resistance is of the order of millions of ohms, although the same metal melted or under pressure would only offer (the dimensions being the same) a resistance equal to a fraction of an ohm. There being, therefore, no current in the circuit, a Leyden jar is discharged at some little distance off, and the abrupt and permanent deflection of the galvanometer needle shows that an immediate and a permanent reduction of the resistance has been caused. The resistance of the metal is no longer to be measured in millions of ohms, but in hundreds. Its conductivity increases with the number and intensity of the sparks.

Some 20 or 30 centimetres from a circuit comprising some metallic filings contained in an ebonite cup, let us place a hollow brass sphere, 15 to 20 centimetres in diameter, insulated by a vertical glass support. The filings offer an enormous resistance and the galvanometer needle remains at zero. But if we bring an electrified stick of resin near the sphere, a little spark will pass between the stick and the sphere, and immediately the needle of the galvanometer is violently jerked and then remains permanently deflected. On some fresh filings being placed in the ebonite cup, the resistance of the circuit will again keep the needle at zero. If now the charged brass sphere is touched with the finger, there is a minute discharge and the galvanometer needle is again deflected. With a few accumulators the experiment can easily be made without a galvanometer. The circuit consists of the battery, some metallic powder, a platinum wire, and a mercury cup. The resistance of the powder is so high that the interruption of the circuit takes place without any sparking at the mercury cup. If now a Leyden jar is discharged in the neighbourhood of the circuit the powder is rendered conducting, the platinum wire immediately becomes red hot, and a violent spark occurs on breaking the circuit.

The influence of the spark decreases as the distance increases, but its influence is observable several metres away from the powder, even with a small Wimshurst machine. Repeating the spark increases the conductivity; in fact, with certain substances successive sparks produce successive jerks, and a gradually increasing and persistent deflection of the galvanometer.

_Influence of a Conductor traversed by Condenser Discharges._--While using a Wimshurst machine it was noticed that the reduction in the resistance of the filings frequently took place before discharge. This led me to the following experiment: Take a long brass tube, one end of which is close to the circuit containing the metallic powder; its other end, several metres distant from the circuit, is fairly close to a charged Leyden jar. A spark takes place and the conductor is charged. At the same instant, the conductivity of the metallic powder is greatly increased.

The following arrangement, owing to its efficacy, convenience, and regularity of action was used by me in most of my researches, and I shall briefly call it the A arrangement (_see_ Fig. 53).

The source of electricity is a two-plate Holtz machine driven at from 100 to 400 revolutions. A sensitive substance is introduced into one of the arms of a Wheatstone bridge, or into the circuit of a single Daniell cell at a distance of some 10 metres (34ft.) from the Holtz machine. Between the discharge knobs of the machine and the Wheatstone bridge, and connected to the former, there are two insulated brass tubes, A A′, running parallel to one another 40 centimetres apart. The Leyden jars usually attached to a Holtz machine may be dispensed with, the capacity of the long brass tubes being in some measure equivalent to them. The knobs S were 1 mm., ·5 mm., or ·1 mm. apart. When the plates were rotated sparks rapidly succeeded each other. Experiments showed that these sparks had no direct effect at a distance of 10 metres. The two tubes A A′ are not absolutely necessary, the diminution of resistance is easily produced if only one is employed, and in some cases, indeed, a single conductor is more efficacious. An increase in the speed of the machine increases its action to a marked extent. The sparks at S may be suppressed by drawing the knobs apart, but the conductor A will still continue to exert its influence, especially if there is a spark gap anywhere about.

_Effects of Induced Currents._--The passage of induced currents _through_ a sensitive substance produces similar effects to those described above. In one instance an induction coil was taken, having two similar wires. The circuit of the secondary wire was closed through a tube containing filings, the galvanometer being also in circuit. Care was taken to ascertain before introducing the filings into the circuit that the currents on make-and-break gave equal and opposite deflections. Filings were then introduced into the circuit, the primary being made and broken at regular intervals. The following table gives the results obtained in the case of zinc filings:--

ZINC FILINGS.

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.