Accordingly, there must be at the receiving station an arrangement called a receiving aerial, which in general takes the form of a long vertical wire or wires, similar in form to the transmitting aerial, There is, however, a distinct difference in the function of the transmitting aerial and the receiving aerial. The function of the first is effective radiation, and for this purpose the aerial must have a.s.sociated with it a store of energy to be released as wave energy; but the function of the receiving aerial is to be the seat of an electromotive force which is created by the electric force and the magnetic force of the incident electric wave.

In tracing out the mode of operation of the transmitting aerial, it was pointed out that the electric waves emitted consisted of alternations of electric force in a direction which is perpendicular to the surface of the earth, and magnetic force parallel to the surface of the earth. These two quant.i.ties, the electric force and the magnetic force, are called the _wave vectors_, and they both lie in a plane perpendicular to the direction in which the wave is travelling and at right angles to one another, the electric force being perpendicular to the surface of the earth. In optical language, the wave sent out by the aerial would be called a plane polarised wave, the plane of polarisation being parallel to the magnetic force. Hence, if at any point in the path of the wave we erect a vertical conductor, as the wave pa.s.ses over it, it is cut transversely by the magnetic force of the wave and longitudinally by the electric force. Both of these operations result in the creation of an alternating electromotive force in the receiving aerial wire.

As in all other cases of oscillatory motion, the principle of resonance may here be brought into play to increase immensely the amplitude of the current oscillations thereby set up in the receiving aerial. As already explained, any vertical insulated wire placed with its lower end near the earth has capacity with respect to the earth, and it has also inductance, the value of these factors depending on its shape and height. Accordingly, it has a natural electrical time-period of its own, and if the periodic electromotive impulses which are set up in it by the pa.s.sage of the waves over it agree in period with its own natural time-period, then the amplitude of the current vibrations in it may become enormously greater than when there is a disagreement between these two periods. Before concluding these articles we shall return to this subject of electric resonance and syntony, and discuss it with reference to what is called the tuning of Hertzian wave stations. Meanwhile, it may be said that for the sake of obtaining, at any rate in an approximate degree, this coincidence of time-period, it is generally usual to make the receiving aerial as far as possible identical with the transmitting aerial. If the receiving aerial is not insulated, but is connected to the earth at its lower end through the primary coil of an oscillation transformer, we can still set up in it electrical oscillations by the impact on it of an electric wave of proper period; and if the oscillation transformer is properly constructed we can draw from its secondary circuit electric oscillations in a similar period.

One problem in connection with the design of a receiving aerial is that of increasing its effective length and capacity so as to increase correspondingly the electromotive force or current oscillations in it.

It is clear that if we put a number of receiving wires in parallel so that each one of them is operated upon by the wave separately, although we can increase in this way the magnitude of the alternating current which can be drawn off from the aerial, we cannot increase the electromotive force in it except by increasing the actual height of the wires. Unfortunately, there is a limit to the height of the receiving aerial. It has to be suspended, like the transmitting aerial, from a mast or tower, and the engineering problem of constructing such a permanent supporting structure higher than, say, two hundred feet is a difficult one.

Since any one station has to send as well as receive, it is usual to make one and the same aerial wire or wires do double duty. It is switched over from the transmitting to the receiving apparatus, as required. This, however, is a concession to convenience and cost. In some respects it would be better to have two separate aerials at each station, the one of the best form for sending, and the other of the best form for receiving.

In Mr. Marconi's early arrangements, the so-called coherer or sensitive wave-detecting appliance, to be described more in detail presently, was inserted between the base of the insulated receiving aerial and the earth, but it was subsequently found by him to be a great improvement to act upon the receiving device, not directly by the electromotive force set up in the aerial, but by the induced electromotive force of a special form of step-up oscillation transformer he calls a "jigger," the primary circuit of which was inserted in between the receiving aerial and the earth plate, and the secondary circuit was connected to the sensitive organ of the telegraphic receiving arrangements.[23] A suggestion to employ transformed oscillations in affecting a coherer, had also been described in a patent specification by Sir Oliver Lodge, in 1897, but the essence of success in the use of this device is not merely the employment of a transformer, but of a transformer constructed specially to transform electrical oscillations.

Turning, then, to the consideration of the relation existing between the transmitting and receiving aerials, we note that in their simplest form these consist of two similar tall rods of metal placed upright, with their feet in good connection with the earth at two places. We may think of them as two identical lightning conductors, well earthed at the bottom, and supported by non-conducting masts or towers. These rods must be in good connection with the earth, and therefore with it form, as it were, one conductor. If, as usual, these aerials are separated by the sea, the intermediate portion of this circuit is an electrolyte. The operations which take place when a signal is sent are as follows:--

At the transmitting station, we set up in the transmitting aerial electric oscillations, of which the frequency may be of the order of a million, _i.e._, the oscillations as long as they last are at the rate of a million a second. Each spark discharge at the transmitter results, however, only in the production of a train of a dozen or two oscillations, and these trains succeed each other at a rate depending upon the transmitting arrangements used. Each oscillation in the transmitting aerial is accompanied by the detachment from it of semi-loops of electric strain, as already explained. The alterations of electric strain directed perpendicularly to the earth, and of the a.s.sociated magnetic force parallel to the earth, const.i.tute an electric wave in the ether, just as the alternations of pressure and motion of air molecules const.i.tute an air wave. a.s.sociated with these physical actions above ground, there is a propagation through the earth of electric action, which may consist in a motion or atomic exchange of electrons. Each change or movement of a semi-loop of electric strain above ground has its equivalent below ground in inter-atomic exchanges or movements of the electrons, on which the ends of these semi-loops of electric strain terminate. The earth must play, therefore, a very important part in so-called "wireless telegraphy," and we might also say the earth does as much as the ether in its production.

The function of the receiving aerial is to bring about a union between these two operations above and below ground. When the electric waves fall upon it, they give rise to electromotive force in the receiving aerial, and, therefore, produce oscillations in it which, in fact, are electric currents flowing into and out of the receiving aerial. We may say that the transmitting aerial, the receiving aerial and the earth form one gigantic Hertz oscillator. In one part of this system, electric oscillations of a certain period are set up by the discharge of a condenser and are propagated to the other part. In the earth, there is a propagation of electric oscillations; in the s.p.a.ce above and between the aerials, there is a propagation of electric waves. The receiving aerial _feels_, therefore, what is happening at the distant aerial and can be made to record it.[24]

We have next to consider the question of the wave-detecting devices which enable us to appreciate and record the impact of a wave or wave train against the aerial. At the very outset it will be necessary to coin a new word to apply generally to these appliances. Most readers are probably familiar with the term "coherer," which was applied by Sir Oliver Lodge, in the first instance, to an electric wave-detecting device of one particular kind--viz., that in which a metal point was lightly pressed against another metal surface and caused to stick to it when an electric wave fell upon it. As our knowledge increased, it was found that there were many cases in which the effect of the electric radiation was to cause a severance and not a coherence, and hence such clumsy phrases as "anticoherer" and "self-decohering coherer" have come into use. Moreover, we have now many kinds of electric wave detectors based on quite different physical principles.

At the risk of incurring reprobation for adding to scientific nomenclature, the author ventures to think that the time has arrived when a simple and inclusive term will be found useful to describe all the devices, whatever their nature, which are employed for detecting the presence of an electric wave. For this purpose the term _k.u.mascope_, from the Greek [Greek: k.u.ma] (a wave), is suggested. The scientific study of waves has already been called _k.u.matology_, and in view of our familiarity with such terms as _microscope_, _electroscope_ and _hygroscope_, there does not seem to be any objection to enlarging our vocabulary by calling a wave-detecting appliance a _k.u.mascope_. We are then able to look at the subject broadly and to cla.s.sify k.u.mascopes of different kinds.

We may, in the first place, arrange them according to the principle on which they act. Thus, we may have electric, magnetic, thermal, chemical and physiological operations involved; and finally, we may divide them into those which are self-restoring, in the sense that after the pa.s.sage or action of a wave upon them they return to their original sensitive condition; and those which are non-restoring, in that they must be subjected to some treatment to bring them back again to a condition in which they are fit to respond again to the action of a wave.

We have no s.p.a.ce to refer to the whole of the steps of discovery which led up to the invention of all the various forms of the modern electric k.u.mascope or wave detector. Suffice it to say that the researches of Hertz in 1887 threw a flood of light upon many previously obscure phenomena, and enabled us to see that an electric spark, and especially an oscillatory spark, creates a disturbance in the ether, which has a resemblance in Nature to the expanding ripples produced by a stone hurled into water. Scientific investigation then returned with fresh interest to previously incomprehensible effects, and a new meaning was read into many old observations. Again and again it had been noticed that loose metallic contacts, loose aggregations of metallic filings or fragments, had a mysterious way of altering their conductivity under the action of electric sparks, lightning discharges and high electromotive forces.

As far back as 1852, Mr. Varley had noticed that ma.s.ses of powdered metals had a very small conductivity, which increased in a remarkable way during thunderstorms;[25] and in 1866, C. and S. A. Varley patented a device for protecting telegraphic instruments from lightning, which consisted of a small box of powdered carbon in which were buried two nearly touching metal points, and they stated that "powdered conducting matter offers a great resistance to a current of moderate tension, but offers but little resistance to currents of high tension."[26]

We then pa.s.s over a long interval and find that the next published account of similar observations was due to Professor T.

Calzecchi-Onesti, who described in an Italian journal, _Il Nuovo Cimento_ (see Vol. XVI., p. 58, and Vol. XVII., p. 38), in 1884 and 1885 his observations on the decrease in resistance of metal powders when the spark from an induction coil was sent through them.[27] These observations did not attract much attention until Professor E. Branly, of Paris, in 1890 and 1891, repeated them on an extended scale and with great variations, making the important observation that an electric spark _at a distance_ had a similar effect in increasing the conductivity of metallic powders.[28] Branly, however, noticed that in some cases of conductors in powder, such as the peroxide of lead or antimony, the effect of the spark was to cause a decrease of conductivity.

To Professor E. Branly unquestionably belongs the honour of giving to science a new weapon in the shape of a tube containing metallic filings or powder rather loosely packed between metal plugs, and of showing that when the pressure on the powder was adjusted such a tube may be a conductor of very high resistance, but that the electrical conductivity is enormously increased if an electric spark is made in its neighbourhood. He also proved that the same effect occurred in the case of two slightly oxidised steel or copper wires laid across one another with light pressure, and that this loose or imperfect contact was extraordinarily sensitive to an electric spark, dropping in resistance from thousands of ohms to a few ohms when a spark was made many yards away.

It is curious to notice how long some important researches take to become generally known. Branly's work did not attract much attention in England until 1892, when Dr. Dawson Turner described his own repet.i.tion of Branly's experiments with the metallic filings tube at a meeting of the British a.s.sociation in Edinburgh. In the discussion which followed, Professor George Forbes made an important remark. He asked whether it was possible that the decrease in resistance could be brought about by Hertz waves.[29]

This question shows that even in 1892 the idea that the effect of the spark on the Branly tube was really due to Hertzian waves was only just beginning to arise. The following year, however, Mr. W. B. Croft repeated Branly's experiment with copper filings before the Physical Society of London, and ent.i.tled his short Paper "Electric Radiation on Copper Filings."[30] He exhibited a tube containing copper filings loosely held between two copper plugs and joined in series with a galvanometer and cell. The effect of an electric spark at a distance, in causing increase of conductivity, was shown, and the return of the tube to its non-conducting state when tapped was also noticed.

In the discussion which followed the reading of this Paper, Professor Minchin described the effects of electric radiation on his impulsion cells. He followed up this by reading a Paper to the Physical Society on November 24, 1893, on the action of Hertzian radiation on films containing metallic powders, and expressed the opinion that the change in resistance of the Branly tube was due to electric radiation.[31]

Thus, at the end of 1893, a few physicists clearly recognised that a new means had been given to us for detecting those invisible ether waves, the chief properties of which Hertz had unravelled with surpa.s.sing skill six years before, by means of a detector consisting of a ring of wire having a small spark-gap in it.

In June, 1894, Sir Oliver Lodge delivered a discourse at the Royal Inst.i.tution, ent.i.tled "The Work of Hertz," and at this lecture use was made of the Branly tube as a Hertz wave detector. The chief object of the lecture was to describe the properties of Hertzian waves and their reflection, absorption and transmission, and many brilliant quasi-optical experiments were exhibited. Although a Branly tube, or imperfect metallic contact, then named by him a _coherer_, was employed by Sir Oliver Lodge to detect an electric wave generated in another room, there was no mention in this lecture of any use of the instrument for telegraphic purposes.[32]

As we are here concerned only with the applications in telegraphy, we shall not spend any more time discussing the purely scientific work done with laboratory forms of this wave detector.

Without attempting to touch the very delicate question as to the precise point at which laboratory research pa.s.sed into technical application, we shall briefly describe the forms of k.u.mascope which have been devised with special reference to Hertzian wave telegraphic work. A very exact cla.s.sification is at present impossible, but we may say that telegraphic k.u.mascopes may be roughly divided into six cla.s.ses. The first cla.s.s includes all those that depend for their action on the "coherer principle" or the reduction of the resistance of a metallic microphone by the action of electromotive force. As they depend upon an imperfect contact, they may be called _contact k.u.mascopes_. This cla.s.s is furthermore subdivided into the self-restoring and the non-self-restoring varieties. The second cla.s.s comprises the _magnetic k.u.mascopes_ which depend upon the action of an electrical oscillation as a magnetising or demagnetising agency. The third cla.s.s comprises the _electrolytic responders_, in which the action of electric oscillations either promotes or destroys the results of electrolysis. The fourth cla.s.s consists of the _electrothermal detectors_, in which the power of an electrical oscillation as a high frequency electric current to heat a conductor is utilised. The fifth cla.s.s comprises the _electromagnetic_ or _electro-dynamic_ instruments, which are virtually very sensitive alternating-current ammeters, adapted for immensely high frequency.

The sixth cla.s.s must be made to contain all those which cannot be well fitted at present into any of the others, such as the sensitive responder of Schafer, the action of which is not very clearly made out.

We may proceed briefly to describe the construction of the princ.i.p.al forms of k.u.mascope coming under the above headings. In the first place, let us consider those which are commonly called the "coherers"

or, as the writer prefers to call them, the _contact k.u.mascopes_. The simplest of these is the crossed needle or single contact, which originated with Professor E. Branly.[33] The pressure of the point of a steel needle against an aluminium plate was subsequently found by Sir Oliver Lodge to be a very sensitive arrangement when so adjusted that a single cell sends little or no current through the contact.[34]

When an electric wave pa.s.ses over it, good conducting contact ensues.

The point is, in fact, welded to the plate, and can only be detached by giving the plate or needle a light shock or vibration. A variation of the above form is a pair of crossed needles, one resting on the other.

Professor Branly found, in 1891, that if a pair of slightly-oxidised copper wires rest across one another the contact-resistance may fall from 8,000 to 7 ohms by the impact of an electric wave. He has recently devised a tripod arrangement, in which a light metal stool with three slightly-oxidised legs stands on a polished plate of steel.

The contact points must be oxidised, but not too heavily, and the stool makes a bad electrical contact until a wave falls upon it.[35]

The decoherence is effected by giving the stool a tilt by means of an electromagnet.

These single or multiple-point k.u.mascopes labour under the disadvantage that only a very small current can be pa.s.sed through the variable contact when used as a relay arrangement, without welding them together so much that a considerable mechanical shock is required to break the contact and reset the trap.

The logical development of the single contact is, therefore, the infinite number of contacts existing in the tube of metallic filings, which has been the form of k.u.mascope most used for many years. In its typical form it consists of a tube of insulating material with metallic plugs at each end, and between them a ma.s.s of metallic powder, filings, borings, granules or small spheres, lightly touching one another. Imperfect contact must be arranged by light pressure, and in the majority of cases the resistance is very large until an electric wave falls upon the tube, when it drops suddenly to a small value and remains there until the tube is given a slight shake or the granules disturbed in any way, when the resistance suddenly rises again. This type of responder is a non-restoring k.u.mascope, and requires the continual operation of some external agency to keep it in a condition in which it is receptive or sensitive to electric waves.

Much discussion and considerable research have taken place in connection with the action and improvement of these metallic powder k.u.mascopes. As regards materials, the magnetic metals, nickel, iron and cobalt, in the order named, appear to give the best results. The n.o.ble metals, gold, silver and platinum, are too sensitive, and the very oxidisable metals too insensitive, for telegraphic work, but an admixture may be advantageously made.

Omitting the intermediate developments of invention, it may be said that Mr. Marconi was the first to recognise that to secure great sensibility in an electric wave detector of this type the following conditions must be fulfilled: An exceedingly small ma.s.s of metallic filings must be placed in a very narrow gap between two plugs, the whole being contained in a vessel which is wholly or partly exhausted of its air. Mr. Marconi devoted himself with great success to the development of this instrument, and in a very short time succeeded in transforming it from an uncertain laboratory appliance, capable of yielding results only in very skilled hands, into an instrument certain and simple in its operations as an ordinary telegraphic relay.

He did this, partly by reducing its size, and partly by a most judicious selection of materials for its construction. As made at present, the Marconi metallic filings tube consists of a small gla.s.s tube, the interior diameter of which is not much more than one-eighth of an inch, which has in it two silver plugs which are bevelled off obliquely. These are placed opposite to each other, so as to form a wedge-shaped gap, about a millimetre in width at the bottom and two, or at most three, millimetres in width at the top (see Fig. 16). The silver plugs exactly fill the aperture of the tube, and are connected to platinum wires sealed through the gla.s.s. The tube has a lateral gla.s.s tube fused into it, by which the exhaustion is made, which is afterwards sealed off, and this tube projects on the side of the wider portion of the gap between the silver plugs. The sensitive material consists of a mixture of metallic filings, five per cent. silver and ninety-five per cent. nickel, being carefully mixed and sifted to a certain standard fineness. In the manufacture of these tubes, great care is taken to make them as far as possible absolutely identical.

Each tube when finished is exhausted, but not to a very high vacuum.

The tube so finished is attached to a bone holder, by which it can be held in a horizontal position. The object of bevelling off the plugs in the Marconi tube is to enable the sensitiveness of the tube to be varied by turning it round, so that the small quant.i.ty of filings lie in between a wider or narrower part of the gap.[36]

[Ill.u.s.tration: FIG. 16.--MARCONI SENSITIVE TUBE OR METALLIC FILINGS k.u.mASCOPE. PP, silver plugs; TT, platinum wires; F, nickel and silver filings.]

Other ways of adjusting the quant.i.ty of the filings to the width of the gap have been devised. Sometimes one of the plugs is made movable.

In other cases, such as the tubes devised by M. Blondel and Sir Oliver Lodge, there is a pocket in the gla.s.s receptacle to hold square filings, from which more or less can be shaken into the gap.

An interesting question, which we have not time to discuss in full, is the cause of the initial coherence of the metallic filings in a Branly tube. It does not seem to be a simple welding action due to heat, and it certainly takes place with a difference of potential, which is very far indeed below that which we know is required to produce a spark. On the other hand, it seems to be proved that in a Banly tube, when acted upon by electric waves, chains of metallic particles are produced. The effect is not peculiar to electric waves. It can be accomplished by the application of any high electromotive force. Thus Branly found that coherence may be produced by the application of an electromotive force of twenty or thirty volts, operating through a very high water resistance, and thus precluding the pa.s.sage of any but an excessively small current. Again, the coherence seems to take place in some cases when metallic particles are immersed in a liquid, or even in a solid, insulator. Processor Branly has, therefore, preferred to speak of ma.s.ses of metallic granules as _radio-conductors_, and Professor Bose has divided substances into positive and negative, according as the operation of electromotive force is to increase the coherence of the particles or to decrease it.

It has been a.s.serted that for every particular Branly tube, there is a critical electromotive force, in the neighbourhood of two or three volts which causes the tube to break down and pa.s.s instantly from a non-conductive to a conductive condition, and that this critical electromotive force may become a measure of the utility of the tube for telegraphic purposes. Thus, C. Kinsley (_Physical Review_, Vol.

XII., p. 177, 1901) has made measurements of this supposed critical potential for different "coherers," and subsequently tested the same as receivers at a wireless telegraph station of the U.S.A. Signal Corps. The average of twenty-four experiments gave in one case 22 volts as the breaking down potential of one of these coherers or Branly tubes, 38 volts for a second and 55 volts for the third.

These same instruments, tested as telegraphic k.u.mascopes, showed that the first of the three was most sensitive.

On the other hand, W. H. Eccles (_Electrician_, Vol. XLVII., pp. 682 and 715, 1901) has made experiments with Marconi nickel-silver sensitive tubes, using a liquid potentiometer made with copper sulphate, to apply the potential so that infinitesimal spark contacts might be avoided and the changes in potential made without any abruptness. He states that if the coherer tube is continuously tapped, say at the rate of fifty vibrations per second, whilst at the same time an increasing potential is applied to its terminals and the current pa.s.sing through it measured on a galvanometer, there is no abrupt change in current at any point. He found that when the current and voltage were plotted against each other, a regular curve was obtained, which after a time becomes linear. A decided change occurs in the conductivity of the ma.s.s of metallic filings when treated in this manner at voltages lower than the critical voltage obtained by previous methods. He ascertained that there was a complete correspondence between the sensitiveness of the tubes used as telegraphic instruments and the form of the characteristic curve of current and voltage drawn by the above-described method.

In the same manner, K. E. Guthe and A. Trowbridge (_Physical Review_, Vol. II., p. 22, 1900) investigated the action of a simple ball coherer formed of half a dozen steel, lead or phosphor-bronze b.a.l.l.s in slight contact. They measured the current _i_ pa.s.sing through the series under the action of a difference of potential _v_ between the ends, and found a relation which could be expressed in the form

v = V(1 - e^{ki}),

where V and _k_ are constants.

The current through this ball coherer is, therefore, a logarithmic function of the potential difference between its ends, of the form

i = log(v - V)

and exhibits no discontinuity.

The inference was drawn that the "resistance" is due to films of water adhering to the metallic particles through which electrolytic action occurs.

A good metallic filings tube for use as a receiver in Hertzian wave telegraphy should exhibit a constancy of action and should cohere and decohere, to use the common terms, sharply, at the smallest possible tap. It should not have a current pa.s.sed through it by the external cell of more than a fraction of a milliampere, or else it becomes wounded and unsensitive.

The investigations which have already taken place seem to show pretty clearly that the agency causing the ma.s.ses of filings to pa.s.s from a non-conductive to a conductive condition is electromotive force, and that, therefore, it is the electromotive force set up in the aerial by the incident waves which is the effective agent in causing the change in the metallic filings tube, when this is used as a telegraphic k.u.mascope.

This transformation of the tube from a non-conductor to a conductor is made to act as a circuit-closer, completing the circuit, by means of which a single cell of a local battery is made to send current through an ordinary telegraph relay, and so by the aid of a second battery operate a telegraphic printer or recorder of any kind. Hence it is clear that after one impact, the metallic filings tube has to be brought back to its non-conductive condition, and this may be achieved in several ways. (1) By the administration of carefully-regulated taps or shocks or by rotating the tube on its axis; (2) by the aid of an alternating current; (3) in those cases where filings of magnetic metals are employed, by magnetism.