Chapter II: Part 2

If a coherer be attached by a side wire to the gas pipes, and an electrophorous spark be given to either the gas pipes or the water pipes or even to the hot-water system in any other room of the building, the coherer responds. It is surprising how far these impulses can be felt along an ordinary uninsulated wire or other conductor.

In fact, when thus connected to gas pipes one day when I tried it, the spot of light could hardly keep still five seconds. Whether there was a distant thunderstorm, or whether it was only picking up telegraphic jerks, I do not know. The jerk of turning on or off an extra Swan lamp can affect it when sensitive. I hope to try for long-wave radiation from the sun, filtering out the ordinary well-known waves by a blackboard or other sufficiently opaque substance.

[I did not succeed in this, for a sensitive coherer
in an outside shed unprotected by the thick walls
of a substantial building cannot be kept quiet for
long. I found its spot of light liable to frequent
weak and occasionally violent excursions, and I could
not trace any of these to the influence of the sun.
There were evidently too many terrestrial sources
of disturbance in a city like Liverpool to make the
experiment feasible. I don’t know that it might not
possibly be successful in some isolated country
place; but clearly the arrangement must be highly
sensitive in order to succeed.]

We can easily see the detector respond to a distant source of radiation now, viz., to a 5 in. sphere placed in the library between secondary coil knobs; separated from the receiver, therefore, by several walls and some heavily gilded paper, as well as by 20 or 30 yards of space (Fig. 19.)

Also I exhibit (Fig. 19B) a small complete detector made by my assistant, Mr. Davies, which is quite portable and easily set up. The essentials (battery, galvanometer, and coherer) are all in a copper cylinder, A, three inches by two. A bit of wire, B, a few inches long, pegged into it, helps it to collect waves. It is just conceivable that at some distant date, say by dint of inserting gold wires or powder in the retina, we may be enabled to see waves which at present we are blind to.

Observe how simple the production and detection of Hertz waves are now. An electrophorus or a frictional machine serves to excite them; a voltaic cell, a rough galvanometer, and a bad contact serves to detect them. Indeed, they might have been observed at the beginning of the century, before galvanometers were known: a frog’s leg or an iodide of starch paper would do almost as well.

A bad contact was at one time regarded as a simple nuisance, because of the singularly uncertain and capricious character of the current transmitted by it. Hughes observed its sensitiveness to sound waves, and it became the microphone. Now it turns out to be sensitive to electric waves, if it be made of any oxidisable medal (not of carbon),[16] and we have an instrument which might be called a micro-something, but which, as it appears to act by cohesion, I at present call a coherer. Perhaps some of the capriciousness of an anathematised bad contact was sometimes due to the fact that it was responding to stray electric radiation. (See Appendix III., pp. 109 and 111.)

[16] FitzGerald tells me that he has succeeded with carbon also. My experience is that the less oxidisable the metal, the more sensitive and also the more troublesome is the detector. Mr. Robinson has now made me a hydrogen vacuum tube of brass filings, which beats the coherer for sensitiveness. July, 1894.

The breaking down of cohesion by mechanical tremor is an ancient process, observed on a large scale by engineers in railway axles and girders; indeed, the cutting of small girders by persistent blows of hammer and chisel reminded me the other day of the tapping back of our cohering surfaces after they have been exposed to the uniting effect of an electric jerk.

_Receiver in Metallic Enclosure._

If a coherer is shut up in a complete metallic enclosure, waves cannot get at it, but if wires are led from it to an outside ordinary galvanometer, it remains nearly as sensitive as it was before (nearly, not quite), for the circuit picks up the waves and they run along the insulated wires into the closed box. To screen it effectively, it is necessary to enclose battery and galvanometer and every bit of wire connection; the only thing that may be left outside is the needle of the galvanometer. Accordingly, here we have a compact arrangement of battery and galvanometer coil and coherer, all shut up in a copper box (Fig. 19C). The galvanometer coil is fixed against the side of the box at such height that it can act conveniently on an outside suspended compass needle. The slow magnetic action of the current in the coil has no difficulty in getting through copper, as everyone knows: only a perfect conductor could screen off that; but the Hertz waves are effectively kept out by the sheet copper.

It must be said, however, that the box must be exceedingly well closed for the screening to be perfect. The very narrowest chink permits their entrance, and at one time I thought I should have to solder a lid on before they could be kept entirely out. Clamping a copper lid on to a flange in six places was not enough. But by the use of pads of tinfoil and tight clamping, chinks can be avoided, and the inside of the box becomes then electrically dark.

If even an inch of the circuit protrudes, it at once becomes slightly sensitive again; and if a mere single wire protrudes through the box, not connected to anything at either end, provided it is insulated where it passes through, the waves will utilise it as a speaking-tube, and run blithely in. And this happens whether the wire be connected to anything inside or not, though it acts more strongly when connected.

In careful experiments, where the galvanometer is protected in one copper box and the coherer in another, the wires connecting the two must be encased in a metal tube (Figs. 19C and 21), and this tube must be well connected with the metal of both enclosures, if nothing is to get in but what is wanted.

Similarly when definite radiation is desired, it is well to put the radiator in a copper hat open in only one direction (Fig. 20), and in order to guard against reflected and collateral surgings running along the wires which pass outside to the exciting coil and battery, as they are liable to do, I am accustomed to put all the sending apparatus in a packing case lined with tinfoil, to the outside of which the sending hat (Fig. 20) is fixed, and to pull the key of the primary exciting circuit by a string from outside, so that not even key connections shall protrude, else exact optical experiments are impossible.

Even then, with the lid of the hat well clamped on, something gets out, but it is not enough to cause serious disturbance of qualitative results. The sender must evidently be thought of as emitting a momentary blaze of light which escapes through every chink. Or, indeed, since the waves are some inches long, the difficulty of keeping them out of an enclosure may be likened to the difficulty of excluding sound; though the difficulty is not quite so great as that, since a reasonable thickness of metal is really opaque. I fancied once or twice I detected a trace of transparency in such metal sheets as ordinary tinplate, but unnoticed chinks elsewhere may have deceived me. It is a thing easy to make sure of as soon as I have more time. (Tinplate is quite opaque. Lead paper lets a little through.)

One thing in this connection is noticeable, and that is how little radiation gets either in or out of a small _round_ hole. A narrow long chink in the receiver box lets in a lot; a round hole the size of a shilling lets in hardly any, unless indeed a bit of insulated wire protrudes through it like a collecting ear trumpet, as at A, Fig. 19C.

It may be asked how the waves get out of the metal tube of an electric gas-lighter. But they do not; they get out through the handle, which being of ebonite is transparent. Wrap up the handle in tinfoil, and a gas-lighter is powerless.

OPTICAL EXPERIMENTS.

And now, in conclusion, I will show some of the ordinary optical experiments with Hertz waves, using as source either one of two devices: either a 5 in. sphere with sparks to ends of a diameter (Fig. 19), an arrangement which emits 7 in. waves but of so dead-beat a character that it is wise to enclose it in a copper hat to prolong them and send them out in the desired direction, or else a 2 in. hollow cylinder with spark knobs at ends of an internal diameter (Fig. 12). This last emits 3 in. waves of a very fairly persistent character, but with nothing like the intensity of one of the outside radiators.

As receiver there is no need to use anything sensitive, so I employ a glass tube full of coarse iron filings, put at the back of a copper hat with its mouth turned well askew to the source, which is put outside the door at a distance of some yards, so that only a little direct radiation can reach the tube. Sometimes the tube is put lengthways in the hat instead of crossways, which makes it less sensitive, and has also the advantage of doing away with the polarising, or rather analysing, power of a crossway tube.

The radiation from the sphere is still too strong, but it can be stopped down by a diaphragm plate with holes in it of varying size clamped on the sending box (right-hand side of Fig. 21).

_Reflection._

Having thus reduced the excursion of the spot of light to a foot or so, a metal plate is held as reflector, and at once the spot travels a couple of yards. A wet cloth reflects something, but a thin glass plate, if dry, reflects next to nothing, being, as is well known, too thin to give anything but “the black spot.” I have fancied that it reflects something of the 3 in. waves.

With reference to the reflecting power of different substances, it may be interesting to give the following numbers showing the motion of the spot of light when 8 in. waves were reflected into the copper hat, the angle of incidence being about 45 deg., by the following mirrors:--

Sheet of window glass 0 or at most 1 division.
Human body 7 divisions.
Drawing board 12 ”
Towel soaked with tap-water 12 ”
Tea-paper (lead?) 40 ”
Dutch metal paper 70 ”
Tinfoil 80 ”
Sheet copper 100 and up against stops.

_Refracting Prism and Lens._

A block of paraffin about a cubic foot in volume is cast into the shape of a prism with angles 75 deg., 60 deg., and 45 deg. Using the large angle, the rays are refracted into the receiving hat (Fig. 21), and produce an effect much larger than when the prism is removed.

An ordinary 9 in. glass lens is next placed near the source, and by means of the light of a taper it is focussed between source and receiver. The lens is seen to increase the effect by concentrating the electric radiation.

_Arago Disc; Grating; and Zone-plate._

The lens helps us to set correctly an 18 in. circular copper disc in position for showing the bright diffraction spot. Removing the disc, the effect is much the same as when it was present, in accordance with the theory of Poisson. Add the lens and the effect is greater. With a diffraction grating of copper strips 2 in. broad and 2 in. apart, I have not yet succeeded in getting good results. It is difficult to get sharp nodes and interference effects with these sensitive detectors in a room. I expect to do better when I can try out of doors, away from so many reflecting surfaces; indoors it is like trying delicate optical experiments in a small whitewashed chamber well supplied with looking-glasses; nor have I ever succeeded in getting clear concentration with this zone-plate having Newton’s rings fixed to it in tinfoil. The coherer, at any rate in a room, does not seem well adapted to interference experiments; it is probably too sensitive, and responds even at the nodes, unless they are made more perfect than is easily practicable. But really there is nothing of much interest now in diffraction effects, except the demonstration of the waves and the measure of their length. There was immense interest in Hertz’s time, because then the wave character of the radiation had to be proved; but every possible kind of wave must give interference and diffraction effects, and their theory is, so to say, worked out. More interest attaches to polarisation, double refraction, and dispersion experiments.

_Polarising and Analysing Grids._

Polarisation experiments are easy enough. Radiation from a sphere, or cylinder, or dumb-bell is already strongly polarised, and the tube acts as a partial analyser, responding much more vigorously when its length is parallel to the line of sparks than when they are crossed; but a convenient extra polariser is a grid of wires something like what was used by Hertz, only on a much smaller scale; say an 18 in. octagonal frame of copper strip with a harp of parallel copper wires (_see_ Fig. 21, on floor). The spark-line of the radiator (Fig. 20) being set at 45 deg., a vertical grid placed over the receiver reduces the reflection to about one-half, and a crossed grid over the source reduces it to nearly nothing.

Rotating either grid a little rapidly increases the effect, which becomes a maximum when they are parallel. The interposition of a third grid, with its wires at 45 deg., between two crossed grids, restores some of the obliterated effect.

Radiation reflected from a grid is strongly polarised, of course, in a plane normal to that of the radiation which gets through it. They are thus analogous in their effect to Nicols, or to a pile of plates.

The electric vibrations which get through these grids are at right angles to the wires. Vibrations parallel to the wires are reflected or absorbed.

_Reflecting Paraffin Surface; Direction of Vibrations in Polarised Light._

To demonstrate that the so-called plane of polarisation of the radiation transmitted by a grid is at right angles to the electric vibration,[17] _i.e._, that when light is reflected from the boundary of a transparent substance at the polarising angle the electric vibrations of the reflected beam are perpendicular to the plane of reflection, I use the same paraffin prism as before; but this time I use its largest face as a reflector, and set it at something near the polarising angle. When the line of wires of the grid over the mouth of the emitter is parallel to the plane of incidence, in which case the electric vibrations are perpendicular to the plane of incidence, plenty of radiation is reflected by the paraffin face. Turning the grid so that the electric vibrations are in the plane of incidence, we find that the paraffin surface set at the proper angle is able to reflect hardly anything. In other words, the vibrations contemplated by Fresnel are the electric vibrations; those dealt with by McCullagh are the magnetic ones.

Thus are some of the surmises of genius verified and made obvious to the wayfaring man.

END OF LECTURE.

[17] _Cf._ Trouton, in _Nature_, Vol. 39, p. 393; and many optical experiments by Mr. Trouton, Vol. 40, p. 398. Since then the above described and depicted apparatus for electro-optic experiments has been imitated in a neat, compact form by Prof. J. Chunder Bose, of Calcutta, and with it he has obtained many admirable and interesting optical results. See _Proc._ Roy. Soc.

NOTE WITH REFERENCE TO ELECTRIC WAVES ON WIRES.

It may be well to explain that in my Royal Institution lecture I made no reference to the transmission of waves along _wires_. I regard the transmission of waves in _free space_ as the special discovery of Hertz; though undoubtedly he got them on wires too. Their transmission along wires is, however, a much older thing. Von Bezold saw them in 1870, and I myself got quantitative evidence of nodes and loops in wires when working with Mr. Chattock in the session 1887-8 (_see_, for instance, contemporary reports of the Bath Meeting of the British Association, 1888, in _The Electrician_), and I exhibited them some time afterwards to the Physical Society, the wires themselves becoming momentarily luminous at every discharge except at the nodes, thus enabling the waves to be actually seen, having been made stationary by reflexion as in the corresponding acoustic experiment of Melde. This experiment does not appear to have been properly known (p. 78).

It may be worth mentioning that the arrangement frequently referred to in Germany by the name of Lecher (viz., that shown in Fig. 23), and on which a great number of experiments have been made, is nothing but a pair of Leyden jars with long wires leading from their outer coats, such as I constantly employed in these experiments. The wires from the outer coat in my experiment were very long, sometimes going five or six times round a large hall, like telegraph wires. And many measurements of wave length were thus made by me at the same period as that in which Hertz was working at Carlsruhe. The use of air dielectric instead of glass permits the capacity to be adjusted, and also readily enables the capacity to be small, and the frequency, therefore, high; but otherwise the arrangement is the same in principle as had frequently been used by myself in the series of experiments called “the recoil kick” (_Proc._ Roy. Soc., June 1891, Vol. 50, pp. 23-39). For these and other reasons no reference has been made in my lecture to the work done on wires by Sarasin and De la Rive; nor to other excellent work done by Lecher, Rubens, Arons, Paalzow, Ritter, Blondlot, Curie, D. E. Jones, Yule, Barton, and other experimenters.

APPLICATION OF THIS METHOD OF SIGNALLING AT A DISTANCE TO ACTUAL TELEGRAPHY.

Although the method of signalling to a moderate distance through walls or other non-conducting obstructions by means of Hertz waves emitted from one station and detected by Branly filing tubes at another station was practised by the author and by several other persons in this country, it was not applied by them to actual telegraphy. The idea of replacing a galvanometer, which was preferably a well-damped or speaking galvanometer, by a relay working an ordinary sounder or Morse was an obvious one, but so far as the present author was concerned he did not realise that there would be any particular practical advantage in thus with difficulty telegraphing across space instead of with ease by the highly developed and simple telegraphic and telephonic methods rendered possible by the use of a connecting wire. In this non-perception of the practical uses of wireless telegraphy he undoubtedly erred. But others were not so blind, though equally busy; and notably Dr. Alexander Muirhead foresaw the telegraphic importance of this method of signalling immediately after hearing the author’s lecture on June 1st, 1894, and arranged a siphon recorder for the purpose. Captain Jackson also, at Devonport, made experiments for the Admiralty, and succeeded in telegraphing between ships in 1895 (or 1896). Prof. Popoff’s telegraphic application in 1895 is mentioned on page 62.

By some chance a knowledge of the coherer method of detecting electric waves did not spread fast in Germany, the many German workers in Hertz waves continuing, for some time after 1894, the older and less efficient, though for metrical purposes often more convenient, mode of detecting them. But, in Italy, the work described in the preceding lecture became well known, and the subject was developed largely, especially by Prof. Righi, of Bologna, in the optical direction. It was also developed in the same direction with many most interesting results by Prof. Bose, of Calcutta, as mentioned in the text. Prof. Righi made a large number of experiments, which he has since described in an Italian treatise, “Opticé Elettrica,” and it appears that it was from him that Signor Marconi learned about the subject, and immediately conceived the idea of applying it to commercial telegraphy. He appears to have worked at the subject for a short time in Italy, aiming at getting the receiver to be more satisfactory and dependable, and improving the early form of Branly filings tube depicted on page 23 by greatly diminishing its size, bringing the terminals closer together, and replacing the coarse borings by fine filings. He also sealed them up in a vacuum, just as the author did, as related on page 34. The only differences, indeed, between his procedure and the author’s during this time were that Signor Marconi preferred nickel filings with a little mercury and a low vacuum, whereas the author adhered chiefly to iron and brass filings and to high vacua. At last he brought it over to Dublin, where he was advised to take it to the Chief of the Government Telegraphs, Mr. Preece, and accordingly he took his, at that time, crude apparatus to the Post Office in a sealed box. There was no point of novelty in it at this stage.

With the powerful aid of the Post Office Signor Marconi proceeded to develop his system of telegraphy on a large scale; and, sometimes failing, sometimes succeeding, gradually increased the distance over which signalling was possible, and especially began to develop from unpromising beginnings his special method for long-distance, viz., the employment of a sending and receiving conducting plate or other small surface, at the top of a lofty pole, connected through what was at that time supposed apparently to be the real radiator, with the earth. This elevated plate, connected as it now is through a simple spark gap with the earth, is an obvious modification of a Hertz vibrator; for it may be regarded simply as a Hertz vibrator with its axis vertical, as Hertz often used it, and with its lower plate replaced by the earth, so as to double the available capacity; but the action of a pair of such elevated plates, connected through the earth conductively and through the air inductively, as now used by Marconi for sender and receiver respectively, is not quite like that of a Hertz vibrator and a Hertz receiver acting on one another by emitted radiation in the ordinary way. If it were not the same earth to which the plates were connected, they would have to act ordinarily by radiation, but since it is the _same_ earth, and that earth conducting (possibly, indeed, with a submerged cable sheath connecting favourably-chosen stations), then the two elevated plates are partially like the greatly separated terminals of a _single_ Hertz vibrator.

Only one of the plates is charged during a sending operation, the other is at zero potential, but some trace of the electrostatic lines from one plate may extend in curved lines to the other, just as they extend to every elevated conductor within hail of the sender in any direction.

Then comes the snap of the spark gap and the sudden discharge, equivalent to the rush of an opposite charge of electricity suddenly into the sending plate, disturbing the electric equilibrium at a distance--at any distance to which any trace of electrostatic field had been able to reach--and giving a kind of what is called in lightning a “_return stroke_.” The effect of this on the distant plate and conductor must be almost infinitesimal; nevertheless, separating it from the earth is the most sensitive detector to a minute sudden rush or jerk of electricity that can be imagined, or that has hitherto been invented,--the coherer. Accordingly, absurdly minute though the disturbance is, the coherer feels it, instantly increases in conductivity, works the relay, and gives the signal. Every spark at the distant spark gap causes a similar rush in or out of the distant elevated plate, and the receiving plate collects such a fraction of this disturbance as to stimulate the coherer and give a signal every time. Not that it is to be supposed never to miss fire. At the present time a coherer is not a rough instrument that can be left free from expert attention with safety for a long time. There are times when it goes on working for days or even weeks, but there are other times when it gives trouble and needs some form of attention. Let us hope that these latter times will become less frequent, and that the whole thing will become quite dependable before long. The pertinacious way in which Mr. Marconi and his able co-operators have, at great expense, gradually worked the method up from its early difficult and capricious stage to its present great distances and comparative dependableness is worthy of all praise.

Telegraphy by means of Hertz waves, though perhaps chiefly developed in this country, has also been pursued successfully by Prof. Slaby in Germany, who has attained considerable distance over land, with its numerous obstacles, and has published an account of his researches in a book called “Funkentelegraphie”; while like success over land has been attained by M. E. Ducretet, M. Blondel and others in France. M. Ducretet has, indeed, put on the market a compact apparatus whereby beginners can readily try their hands at this mode of signalling; as well as a large-scale apparatus like that employed by Lieutenant Tissot for lighthouse signalling on the coast of Brittany.

The filings tube now chiefly employed by the author is of the following form:--It is a sealed glass tube containing carefully selected iron filings, and exhausted to the highest vacuum. Close together are two little silver globes melted each on its own platinum wire terminal, which are connected with convenient screws on an ebonite stand. The filings are adjusted so as just to cover the two silver globes, and no more; a pocket, or reservoir, however, is sometimes provided whereby more or fewer filings can be easily introduced into the working compartment for experimental purposes. This pocket serves to fix the whole tube to its ebonite body, which is provided with a clamp to attach it to the stiff spring, or movable lever, or other form of support, through which it is to receive the mechanical shocks necessary to restore or decohere it after an electrical stimulus.

The usual plan is to employ an electrical hammer to rap strongly on a stiff brass spring to which the ebonite is clamped, but another plan is to attach the coherer to a lever tilted strongly by an electromagnet after the fashion of a sounder. A rapid succession of gentle taps is often better than one violent one, but experience is the best test of the kind of restoration wanted, for it depends a good deal on the strength of the electrical stimulus. There are methods of dispensing with this decohering operation altogether.

After a fairly strong electric stimulus all the filings are stuck together into a sort of mat, and nothing but a thorough shaking up will pull them asunder again. A still more violent electric shock may indeed have a decohering effect, but it is not a plan to be recommended, for it seems to be a heat effect, akin to the blowing of a fuse.

For protecting a coherer from undesired stimuli, _e.g._, from the radiator at its own station, the general method is described on page 35, &c., and the details of it, with the necessary switch for changing over from sending to receiving, are mentioned further on (page 60). But by referring to page 106 it will be seen that M. Branly had already employed such a protecting case, and had worked details out admirably.

Recently Signor Tommasina has shown that, if one end of a short rod or wire be dipped into filings while sparks are occurring in the neighbourhood, the filings adhere to it and to each other, and with care a long string of them can be picked up. The author has examined the behaviour of filings under electrical influence on a glass plate in a microscope, and their movements towards the formation of a complete conducting bridge between the tinfoil terminals together with their disjunctive behaviour when the electrical stimulus is too strong, the thorough cohesion set up by a succession of electrical stimuli, and the partial or complete disruption by an appropriate mechanical stimulus is instructive.

An earlier and most important telegraphic application, based upon information given in the preceding lecture, was made in 1895 by Prof. Popoff, of Russia, and will be mentioned shortly (_see_ page 62). I now proceed to developments of syntonic or attuned telegraphy on the true Hertz-wave principle, the preliminary experiments on which are mentioned above in connection with the figures on page 27.

FURTHER DEVELOPMENTS IN THE TELEGRAPHIC DIRECTION.

SYNTONIC TELEGRAPHY.

In the present state of the law in this country it appears to be necessary for a scientific man whose investigations may have any practical bearing to refrain from communicating his work to any scientific society, or publishing it in any journal until he has registered it and paid a fee to the Government under the so-called Patent Law. This unfortunate system is well calculated to prevent scientific men in general from giving any attention to practical applications, and to deter them from an attempt to make their researches useful to the community. If a scientific worker publishes in the natural way, no one has any rights in the thing published; it is given away and lies useless, for no one will care to expend capital upon a thing over which he has no effective control. In this case practical developments generally wait until some outsider steps in and either patents some slight addition or modification, or else, as sometimes happens, patents the whole thing, with some slight addition. If a scientific worker refrains from publishing and himself takes out a patent, there are innumerable troubles and possible litigation ahead of him, at least if the thing turns out at all remunerative; but the probability is that, in his otherwise occupied hands, it will not so turn out until the period of his patent right has expired.

Pending a much-to-be-desired emendation of the law, whereby the courts can take cognisance of discoveries or fundamental steps in an invention communicated to and officially dated by a responsible scientific society, and can thereafter award to the discoverer such due and moderate recompense as shall seem appropriate when a great industry has risen on the basis of that same discovery or fundamental invention--pending this much-to-be-desired modification of the law, it appears to be necessary to go through the inappropriate and repulsive form of registering a claim to an attempt at a monopoly. The instinct of the scientific worker is to publish everything, to hope that any useful aspect of it may be as quickly as possible utilised, and to trust to the instinct for fair play that he shall not be the loser when the thing becomes commercially profitable. To grant him a monopoly is to grant him a more than doubtful boon; to grant him the privilege of fighting for his monopoly is to grant him a pernicious privilege, which will sap his energy, waste his time, and destroy his power of future production.

However, the author, in consultation with friends, decided that registration was, under present conditions, necessary, and, accordingly, for his attempt at syntony and other improvements in the Hertz wave method of signalling, he can refer here to certain patents taken out, in conjunction chiefly with Dr. Alexander Muirhead, his co-worker, which are numbered respectively as follows:--

(1) 11,575 of 1897, wherein is described the general syntonic principle and the mode of prolonging the duration of the vibrations emitted by a radiator or by a receiver. This is done by adding to it electromagnetic inertia (that is, a self-induction coil) in such a way as to lessen its radiating power, converting its type of emission from something like a whip-crack into something more like that of a struck string. (Not pushing it so far as to make it like a _fork_, though that could be done if desired: see _Journal_ Inst.E.E., December, 1898.) But too prolonged a duration of vibration is not desirable, for it can only be obtained at the expense of radiating power. For the most distant signalling the single pulse or whip-crack is the best, and this is what in practice has hitherto always been employed; but, with it, tuning is of course impossible. A radiator with several swings is less violent at its first impulse than is a momentary emitter; but then the lessened emitting power of a radiator is to be compensated by a correspondingly prolonged duration of vibration on the part of the receiver or absorber, thus rendering the radiator susceptible of tuning to a special similarly-tuned receiver or resonator. The tuned resonator is then to respond, not to the first impulse of the radiator, but to a rapidly worked up succession of properly timed impulses; so that at length, after an accumulation of two or three, or perhaps four, swings, the electrostatic charges in its terminal plates become sufficient to overflow and spit off into the coherer, thereby effecting its stimulation and giving the signal. A resonator not properly tuned--_i.e._, one tuned to some different frequency of vibration--would not be able to accumulate impulses, and hence would not respond, unless of course it were so much too near the radiator that the very first swing stimulated it sufficiently to disturb the coherer; in which case, again, there is no room for tuning. The two points to attend to for syntonic discrimination are: (_a_) that the receiver shall not be so near the emitter as to feel its impulses too easily, _i.e._, without accumulation; (_b_) that the properly tuned receiver shall be so arranged that it can work up and accumulate the impulses of the radiator, and before attaining its maximum swing can overflow into the coherer associated with it and thus give the signal.

The general appearance of a pair of signalling stations on this plan is shown in Fig. 24, where the huts contain the sending and receiving instruments. The self-induction coil joining the two capacity-areas is better depicted in Fig. 25, which also shows one mode of joining up the coherer to a syntonic receiver. (The galvanometer and shunt are, of course, merely typical of any kind of telegraphic instrument whatever.) Fig. 26 indicates one form of sender with three alternative syntonising coils for speaking to three distant attuned stations. Fig. 27 shows a radiator arranged for receiving, but illustrates another method of charging, and one frequently employed by the author, viz., the method by impulsive rush (compare Figs. 11, 12 and 19, on pp. 14 and 25 of this book). The terminals of the Ruhmkorff coil are here connected, not to the capacity-areas direct, but to a pair of knobs near the centre of gravity of each area, so that when the discharge occurs each area is suddenly charged oppositely, and the two opposite charges are left to surge into one another and set up the oscillations. This impulsive method of charging is essentially that adopted in the spherical whip-crack emitter depicted in Fig. 19 (p. 25, _ante_), the two poles of the sphere having but small capacity and being joined by as thick a conductor as the equator of the sphere. But for such a radiator as is indicated in Fig. 24 or Fig. 27 the author commonly found that a third short spark gap in the middle was an improvement, and so, as is well known, did Prof. Righi find it, and embodied it in his well-known double-sphere double-knob emitter.

The specification also contains figures of earth-connected forms of radiators, with or without self-induction coils, of which Fig. 28 may be here reproduced; and likewise a modification of the point coherer depicted in Fig. 17, on page 22 (_see_ Fig. 29, and also fig. on page 27), where the spiral wire spring is replaced by a piece of straight watch-spring, clamped at one end, adjusted by a screw at the other, and lightly touched by a needle point at its middle; a very gentle tapping back stimulus being provided in the form of a clockwork or other mechanically-driven motor grazing lightly against one end of the spring protruding beyond the clamp for the purpose.

Fig. 30 shows a coherer inserted in a secondary or transformer circuit, and operated inductively by the oscillations of the receiver, which are thus transformed up and raised in potential.

(2) No. 16,405, 1897, wherein are described chiefly various practical methods of decohering, by means of cams and otherwise, which are appropriate when working rapidly with automatic transmitter and siphon recorder.

(3) No. 18,644, 1897, represents different ways of connecting up a coherer to a syntonic resonator, so as to get the benefit of its overflow without interfering with the working up of the electric oscillations, _e.g._, Figs. 31, 32 and 33. It also shows a plan for constantly decohering by a rapidly revolving cam a number of coherers in parallel, so that one at least is always ready to receive an impulse (Fig. 34). Further, it arranges to utilise the earth or a cable sheath, or other uninsulated conductor, for the purpose of conveying electric impulses to a distance (Figs. 35, 36, 37 and 38). And next it is arranged to assist the coherer to feel the full effect of any electric jerk by shunting out the battery and galvanometer, which are necessarily in series with it, by means of a condenser of moderate capacity (Fig. 35), which also shows a self-induction mode of sending a stimulus along an uninsulated line. This condenser obstructs all steady currents, such as give the signal, but it transmits freely any momentary electric impulses, such as stimulate a coherer.

(4) No. 29,069, 1897. In this patent various methods of connecting up the shunting condenser, whose object it is to transmit all jerks undiluted to the coherer, are shown, all adapted to work with a syntonic resonator (Fig. 39). There is also shown a complete switch (Fig. 40) for effecting the transition from “sending” to “receiving,” exposing the coherer to the full effect of the distant radiator, and completely protecting and isolating it from its home radiator; the switch being so arranged that signalling is impossible unless the home coherer is protected. A rotating commutator is also shown, whose object is to expose the coherer to the full influence of a receiver, especially of a non-syntonic receiver or simple collector, without its being shunted or otherwise interfered with by the telegraphic apparatus; to which, however, immediately afterwards the rotating commutator connects it, and then effects the tapping back.

Connections are shown (Fig. 41) for a complete sending and receiving station on this plan with a syntonic radiator and resonator indicated (though not to scale). But with syntonic resonators the revolving commutator method is not found to be necessary; the sending and receiving switch, together with the closed box for protecting the coherer in an instantly accessible manner is therefore the chief feature of this diagram.

EARLIER TELEGRAPHIC ADVANCES.

In April, 1895, a communication was made to the Russian Physical Society by Prof. A. Popoff, of the Torpedo School, Cronstadt, Russia, and appears in the _Journal_ of that Society for January, 1896. In this communication the use of an elevated wire and of a tapper-back worked through a relay by the coherer current are clearly described, and signalling was effected for a distance of 5 kilometres (3½ miles).

An extract from this communication is given in _The Electrician_ for December, 1897, Vol. XL., page 235, and from it we reproduce Fig. 42, illustrating the tapping back arrangement.

The following extracts from this paper may also be quoted:--

“On using a sensitive relay in the circuit with the coherer tube, and an ordinary electric bell in the other circuit of the relay, for sound signals and as an automatic tapper for the coherer, I obtain an apparatus which exactly answers every electric wave by a short ring, and by rhythmical strokes if electric vibrations be excited continuously.”

“On connecting an electromagnetic recorder in parallel with the bell, tracing a straight line along the paper band which is moved by a 12-hour clockwork cylinder, I obtain an instrument registering by a cross line on the moving band every electric wave that reaches the coherer from across the atmosphere. Such an apparatus was placed at the Meteorological Observatory at St. Petersburg in July, 1895, one of the electrodes of the coherer being connected by an insulated wire with an ordinary lightning conductor, the other electrode of the tube-coherer being connected with the ground.”

Prof. Popoff then goes on to say that his apparatus works well as a lightning recorder, and that he hopes it can be used for signalling to great distances. He says:--

“I can detect waves at the distance of one kilometre if I employ as sender a Hertz vibrator with 30 centimetre spheres, and if I use the ordinary Siemens relay; but with a Bjerknes vibrator 90 centimetres diameter, and a more sensitive relay, I reach five kilometres of good working.”

Thus it is plain that Prof. Popoff employed the elevated wire as receiver in 1895, but did not employ it as sender.

In 1897 Prof. Slaby, of Berlin, published (in German) a book called “Spark Telegraphy,” in which he described his success in signalling from 3 to 13 miles across land. From this book we take the following illustrations of the coherer and its connections:--

Fig. 43 shows the coherer tied on to a glass tube, by which it is supported.

Fig. 44 shows the simplest form of its connection to a one-cell battery A and a polarised relay B, which switches on another battery of several cells _a_ operating the Morse instrument or electric bell or sounder _b_ and also the tapper-back _c_, the hammer of which raps gently on the coherer tube at every signal.

The actual apparatus is depicted in two views, Figs. 45 and 46, where will be recognised on the left-hand side the coherer and tapper-back; in the middle the batteries, both for relay and for coherer circuits; and on the right-hand side a relay and the signalling or calling instrument, in this case shown as an ordinary electric bell.

A Morse instrument is to be connected to the terminals M, and either it or the bell can be switched into the circuit at pleasure. The form of relay depicted is special to Slaby, but the rest of the arrangements are practically identical with those shown by Marconi at Dover.

Fig. 47 gives a diagram of the actual connections.

Fig. 48 is a picture of one of Slaby’s signalling stations, showing the way the elevated wire enters the building.

During September, 1899, the Marconi method of signalling to long distances was demonstrated before the British Association at Dover. The chief feature of the installation was the elevated wire supported by a mast, and terminating at the top in a small conductor, which is usually made of wire netting, and is suspended from an insulating rod. The lower end of this elevated wire passed into the building through an aperture, and was connected to one terminal of the usual Ruhmkorff coil, the other terminal of which was earthed. The signalling key was of the simplest description, being nothing more than a well-insulated Morse key worked by hand and causing a make-and-break in the primary circuit of the coil. The ordinary trembling break of the induction coil was at work in the usual way, so that while the signalling key was depressed continuously there was a torrent of sparks between the knobs of the secondary. This method of signalling was identical with that employed by everyone since the time of Hertz, except that, instead of connecting the secondary terminals to two insulated plates, one was now connected to earth and the other to a small insulated conductor at considerable elevation.

From this mast in the town of Dover (Fig. 49) signals could be sent to another loftier mast at the South Foreland (Fig. 50), where it is itself elevated by chalk cliffs far above the sea. From this South Foreland station, which was similar in all essential respects to the Dover station, except that its elevation was greater, messages could be sent and received to and from a station near Boulogne, on the coast of France, and to and from the East Goodwins lightship. The signalling was slow, but appeared dependable, and the simplicity of all the arrangements was remarkable (Fig. 51). Concerning the receiving apparatus there is little to be said, since it is in essence the same as that which has already been described. It consists of a coherer of the plug tube pattern, something like that depicted on page 23, but excessively reduced in size, the glass tube being the size of a quill, the two silver plugs close together separated only by a very few nickel filings. This tube is mounted so that it can be struck after each signal by a light electric hammer worked by a current from a local battery switched on by a Siemens’ polarised relay, which is itself actuated by the coherer current. Whenever the coherer receives a signal the same current that works the tapper works also the Morse instrument standing on the table alongside, and records a short or a long signal on the tape. The coherer with its tapper, the polarised relay, and the battery (a few dry cells) are all enclosed in one oblong iron box, through an aperture in which the lower end of the elevated wire can be inserted and brought into direct connection with the coherer.

To change from transmitting to receiving nothing is needed but the detachment of this wire from the Ruhmkorff coil terminal and its insertion through the aperture of the enclosing box so as to touch the coherer circuit. The object of the box is, of course, the protection of the coherer from undesired disturbances, exactly as described on page 34, and the collecting wire has the function there described likewise.

The electric tapper-back is also mentioned on page 31, but not as being operated through a relay by the coherer circuit’s own current. This last improvement seems to have been devised and employed by Prof. Popoff at Cronstadt in 1895 (_see_ Fig. 42). No doubt it was arrived at independently again by Mr. Marconi and the telegraph officials who assisted him in his early experiments in this country.

The other box shown in Fig. 51 is probably a stand-by in case of accident.

It is difficult to imagine a simpler contrivance, and it appeared to work at Dover dependably, the messages coming out slowly in ordinary dots and dashes, the torrent of sparks being sufficiently rapid not to necessitate the breaking up of the dash into a series of dots. The sluggishness of the Morse instrument or the relay, or the circuit as a whole, enabled this excellent result to be attained with apparent ease.

A diagram of Marconi’s connection of sensitive tube to the relay and tapper-back and Morse instrument, where W represents the elevated wire, is given in Fig. 52.

The mast at the South Foreland was stated to be 150 ft. high, but the cliff on which it stands must be at a still greater elevation above the sea. It was from this station that the real distant signalling was performed, and probably not from the lower mast at Dover.

THE HISTORY OF THE COHERER PRINCIPLE.

_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.