Signalling across space without wires / being a description of the work of Hertz & his successors

I picture the action as follows: Suppose two fairly clean pieces of metal in light contact—say two pieces of brass or of iron—connected to a single voltaic cell; a film of what may be called oxide intervenes between the surfaces so that only an insignificant current is allowed to pass, because a volt or two is insufficient to break down the insulating film, except perhaps at one or two atoms.[14] If the film is not permitted to conduct at all, it is not very sensitive; the most sensitive condition is attained when an infinitesimal current passes, strong enough just to show on a moderate galvanometer.

Now let the slightest surging occur, say by reason of a sphere being charged and discharged at a distance of forty yards; the film at once breaks down, perhaps not completely—that is a question of intensity—but permanently. As I imagine, more molecules get within each other’s range, incipient cohesion sets in and the momentary electric quiver acts somewhat like a flux. It is a singular variety of electric welding. A stronger stimulus enables more molecules to hold on, the process is surprisingly metrical; and as far as I roughly know at present, the change of resistance is proportional to the energy of the electric radiation, from a source of given frequency.

It is to be specially noted that a battery current is not needed to effect the cohesion, only to demonstrate it. The battery can be applied after the spark has occurred, and the resistance will be found changed as much as if the battery had been on all the time.

The incipient cohesion electrically caused can be mechanically destroyed. Sound vibrations, or any other feeble mechanical disturbances, such as scratches or taps, are well adapted to restore the contact to its original high resistance sensitive condition. The more feeble the electrical disturbance the slighter is the corresponding mechanical stimulus needed for restoration. When working with the radiating sphere (Fig. 19) at a distance of forty yards out of window, I could not for this reason shout to my assistant to cause him to press the key of the coil and make a spark, but I showed him a duster instead, this being a silent signal which had no disturbing effect on the coherer or tube of filings. I mention 40 yards, because that was one of the first outdoor experiments; but I should think that something more like half-a-mile was nearer the limit of sensitiveness for this particular apparatus as then arranged. However, this is a rash statement not at present verified.[15] At 40 or 60 yards the exciting spark could be distinctly heard, and it was interesting to watch the spot of light begin its long excursion and actually travel a distance of 2 in. or 3 in. before the sound arrived. This experiment proved definitely enough that the efficient cause travelled quicker than sound, and disposed completely of any sceptical doubts as to sound waves being, perhaps, the real cause of the phenomenon. Signals were obtained across the full width of the college quadrangle, and later, with larger apparatus, between the college tower and another high building half-a-mile away.

Invariably, when the receiver is in good condition, sound or other mechanical disturbance acts one way, viz., in the direction of increasing resistance, while electrical radiation or jerks act the other way, decreasing it. While getting the receiver into condition, or when it is getting out of order, vibrations and sometimes electric discharges act irregularly; and an occasional good shaking does the filings good. I have taken rough measurements of the resistance by the simple process of restoring the original galvanometer deflection by adding or removing resistance coils. A half-inch tube, 8 in. long, of selected iron turnings (Fig. 18) had a resistance of 2,500 ohms in the sensitive state. A feeble stimulus, caused by a distant electrophorus spark, brought it down 400 ohms. A rather stronger one reduced it by 500 and 600, while a trace of spark given to a point of the circuit itself ran it down 1,400 ohms.

This is only to give an idea of the quantities. I have not yet done any seriously metrical experiments.

Added later.—My assistant, Mr. E. E. Robinson, early noticed that when a telephone was used as receiver, say with a single-point coherer (see illustration on opposite page), which is a very sensitive arrangement, every disturbance of the coherer due to received waves is accompanied by a crackle or tick in the telephone, without any tapping back being necessary. This is, indeed, the easiest mode of receiving signals, and we often practised it. If a suitable, well-damped galvanometer, such as a Thomson marine speaking-galvanometer, is included also in the circuit (a more sensitive one is sometimes necessary—and we frequently used a D’Arsonval—but it must be well damped), the meaning of these ticks is recognised; each represents a minute change in the resistance of the coherer—not at all the full change usually employed, but little subsidiary changes, sometimes up and sometimes down, barely sufficient to affect a galvanometer, but quite adequate (being so sudden) to disturb a telephone. This method of receiving, which at first is very sensitive, after a time becomes less so; the point shows signs of fatigue, probably due to too perfect cohesion having been gradually established, and a mechanical tap back is desirable to restore it to its original condition.

If all the signals received were precisely of the same strength, I doubt if these superposed crimples of resistance would occur; but signals depending on quality of sending spark never are of the same strength, and accordingly the sudden slight variations of resistance do occur. Usually an ordinary high resistance telephone was employed, and it was joined to the coherer circuit through one of the usual small transformers—a plan which has many obvious advantages.

The fluctuations of resistance of a coherer dependent on various strengths of stimulus are instructively shown in some metrical experiments made by Mr. Robinson, and a plotting of which I showed to the Physical Society of London in 1897. This plotting is here reproduced, and it shows the singular fact that, whereas a stronger electrical stimulus usually decreases the resistance, as is natural, a weaker subsequent stimulus usually increases it again: so that alternately strong and weak stimuli send the curve zigzagging up and down, until it gets into a condition demanding rejuvenation by a mechanical tap back.

Sometimes a decidedly strong electrical stimulus knocks down the conductivity of the coherer as if it had been tapped back. This is almost certainly due to a burning of the delicate contacts—a blowing of a fuse as it were,—and the effect of this electrical burn back is quite different from the effect of a mechanical tap back, inasmuch as it leaves the coherer insensitive. A shaking up is necessary to restore it.

I now call your attention to the Table on next page of various kinds of detector for electric radiation distributed in groups.

Selenium is inserted in this table in the microphone column, because it is a substance which in certain states is well known to behave to visible light as these other microphonic detectors behave to Hertz waves. It is inserted with a query, to indicate that its position in the table is not certainly known. It may possibly belong to some other column.

Electrical Theory of Vision.

And I want to suggest that quite possibly the sensitiveness of the eye is of the coherer kind. As I am not a physiologist, I cannot be seriously blamed for making wild and hazardous speculations in that region. I therefore wish to guess that some part of the retina is an electrical organ, say like that of some fishes, maintaining an electromotive force which is prevented from stimulating the nerves solely by an intervening layer of badly conducting material, or of conducting powder with gaps in it; but that when light falls upon the retina these gaps become more or less conducting, and the nerves are stimulated. I do not feel clear which part is taken by the rods and cones, and which part by the pigment cells; I must not try to make the hypothesis too definite at present, though I hope it is obvious what I intend to suggest.

If I had to make a demonstration model of the eye on these lines, I should arrange a little battery to excite a frog’s nerve-muscle preparation through a circuit completed all except a layer of filings or a single bad contact. Such an arrangement would respond to Hertz waves. Or, if I wanted actual light to act, instead of grosser waves, I would use a layer of selenium.

But the bad contact and the Hertz waves are the most instructive, because we do not at present really know what the selenium is doing, any more than what the retina is doing.

And observe that (to my surprise, I confess) the rough outline of a theory of vision thus suggested is in accordance with some of the principal views of the physiologist Hering. The sensation of light is due to the electrical stimulus; the sensation of black is due to the mechanical or tapping back stimulus. Darkness is physiologically not the mere cessation of light. Both are positive sensations, and both stimuli are necessary; for until the filings are tapped back vision is persistent. In the eye model the period of mechanical tremor should be, say, ⅒th second, so as to give the right amount of persistence of impression.

No doubt in the eye the tapping back is done automatically by the tissues, so that it is always ready for a new impression, until fatigued. And by mounting an electric bell or other vibrator on the same board as a tube of filings, it is possible to arrange so that a feeble electric stimulus shall produce a feeble steady effect, a stronger stimulus a stronger effect, and so on; the tremor asserting its predominance, and bringing the spot back, whenever the electric stimulus ceases.

An electric bell thus close to the tube is, indeed, not the best vibrator; clockwork might do better, because the bell contains in itself a jerky current, which produces one effect, and a mechanical vibration, which produces an opposite effect, hence the spot of light can hardly keep still. By lessening the vibration—say, by detaching the bell from actual contact with the board, the electric jerks of the intermittent current drive the spot violently up the scale; mechanical tremor brings it down again. It must be clearly understood that electric jerks, due to the make-and-break of an ordinary current, are quite adequate to electrically stimulate a coherer in their neighbourhood. It is constantly to be noticed that a coherer responds best to excessively short sparks of a certain sharp quality.

You observe that the eye on this hypothesis is, in electrometer language, heterostatic. The energy of vision is supplied by the organism; the light only pulls a trigger. Whereas the organ of hearing is idiostatic. I might draw further analogies between this arrangement and the eye, e.g., about the effect of blows or disorder causing irregular conduction and stimulation, of the galvanometer in the one instrument, of the brain cells in the other.

A handy portable exciter of electric waves is one of the ordinary hand electric gas-lighters, containing a small revolving doubler—i.e., an inductive or replenishing machine. A coherer can feel a gas-lighter across a lecture theatre. Minchin often used them for stimulating his impulsion cells. I find that when held near they act a little even when no ordinary spark occurs, plainly because of the little incipient sparks at the brushes or tinfoil contacts inside. A Voss machine acts similarly, giving a small deflection while working up before it sparks: indeed, these small sparks are often more effective than bigger ones.

Demonstration of Ordinary Holtz Machine
Sparks not exciting Tube:
except by help of a polished knob.

And notice here that our model eye has a well-defined range of vision. It cannot see waves too long for it. The powerful disturbance caused by the violent flashes of a Holtz or Wimshurst or Voss machine it is blind to. The loud sparks have no effect on it. They are like infra-red radiation to the eye. If the knobs of the machine are well polished the coherer begins to respond again, evidently by reason of some high harmonics, due to vibrations in the terminal rods; and these are the vibrations to which it responds when excited simply by an induction coil. The coil should have knobs instead of points. Sparks from points or dirty knobs hardly excite the coherer at all. But hold a well-polished sphere or third knob between even the dirty knobs of a Voss machine, and the coherer responds at once to the surgings got up in that clean sphere.

Feeble short sparks again are often more powerful exciters than are strong long ones. I suppose because they are more sudden. This is instructively shown with an electrophorous lid. Spark it to a knuckle, and it does very little. Spark it to a clean knob held in the hand and it works well. But now spark it to an insulated sphere, there is some effect. Discharge the sphere, and take a second spark, without recharging the lid; do this several times; and at last, when the spark is inaudible, invisible, and otherwise imperceptible, the coherer some yards away responds more violently than ever, and the spot of light rushes from the scale.

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.)

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:—

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.

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.