The decoherence by taps was discovered by Branly,[37] and Popoff, following the example of Sir Oliver Lodge, employed an electric bell arrangement for this purpose.[38]

Mr. Marconi, in his original receiving instruments, placed an electromagnet under the coherer tube with a vibrating armature like an electric bell.[39] This armature carries a small hammer or tapper, which, when set in action, hits the tube on the under side, and various adjusting screws are arranged for regulating exactly the force and amplitude of the blows. This tapper is actuated by the same current as the Morse printer, or other telegraphic recorder, so that when the signal is received and the metallic filings tube pa.s.ses into the conductive condition and closes the relay circuit, this latter in turn closes the circuit of the Morse printer or other recorder, and at the same time, a current pa.s.ses through the electromagnet of the tapper and the tube is tapped back. This sequence of operations requires a certain time which limits the speed of receiving. The tapper has to be so arranged that it is possible to receive and to record not only the _dot_ but a _dash_ on the Morse system. The _dash_ is really a series of closely adjacent dots, which run together in virtue of the inertia and inductance of the different parts of the whole receiving apparatus. The adjustment has so to be made that, whilst the _dash_ is being recorded and a continuous tapping is kept up, yet, nevertheless the continued electromotive force in the aerial, due to the continually arriving trains of waves, is able to act against the tapping and to keep the filings in the tube in the conductive condition. Hence, the successful operation of the arrangement requires attention to a number of adjustments, but these are not more difficult, or even as difficult, as those involved in the use of many telegraphic receivers employed in ordinary telegraphy with wires.

Mr. Marconi also introduced devices for preventing the sparks at the contacts of the electromagnetic hammer from directly affecting the tube, and also to prevent the electric oscillations which are set up in the aerial from being partly shunted through other circuits than that of the sensitive tube. We pa.s.s on to notice the remaining devices for restoring the metallic filings tube to a condition of sensitiveness or receptiveness.

A method for doing this by alternating currents is due to Mr. S. G.

Brown.[40] The pole pieces of the coherer tube are made of iron, and they are enveloped in magnetising coils traversed by an alternating electric current. Between these pole pieces is placed a small quant.i.ty of nickel or iron filings, and under the action of the electromotive force, due to an electric wave acting on them, may be made to cohere in the usual fashion; but the moment that the wave ceases, the alternating magnetism of the electrodes causes the filings to drop apart or decohere. In place of the alternating current, Mr. Brown finds that a revolving permanent magnet can be used to produce the alternating magnetisation of the pole pieces of the sensitive tube or coherer.

The third method of causing the decoherence of the filings is that due to T. Tommasina. He found that when a Branly tube is made with filings of a magnetic metal, such as iron, nickel and cobalt, the decoherence of the filings can be produced by means of an electromagnet placed in a suitable position under the tube.[41] The explanation of this fact seems to be that, when an electric wave falls upon the tube or when any other source of electromotive force acts upon it, chains of metallic particles are formed, stretching from one electrode to the other. Tommasina contends that he has proved the existence of these chains of particles by experiments made with iron filings; and R.

Malagoli,[42] in referring to Tommasina's a.s.sertion, states that it can be witnessed in the case of bra.s.s filings placed between two plates of metal and immersed in vaseline oil, when a difference of potential is made between the plates.

T. Sundorph[43] says he has confirmed Tommasina's discovery of the formation of these chains of metallic particles in the coherer. The filings do not all cling together, but certain chains are formed which afford a conducting path for the current subsequently pa.s.sed through the coherer from an external source. Accordingly, Tommasina's method of causing decoherence in the case of filings of magnetic metals is to pull them apart by an external magnetic field; and he stated that decoherence can be effected more easily and regularly in this way than by tapping. Whilst on this point, it may be mentioned that C.

Tissot[44] says that he has found that the sensitiveness of a coherer formed of nickel and iron filings can be increased by placing it in the magnetic field, the lines of which are parallel to the axis of the tube. According to MM. A. Blondel and G. Dobkevitch, this is merely the result of an increased coherence of the particles.

Quite recently, Sir Oliver Lodge and Dr. Muirhead have employed as a self-restoring coherer or k.u.mascope a steel disc revolved by clockwork, the edge of which just touches a globule of mercury covered with a thin film of paraffin oil. The contact is made between the mercury and the steel by the electric wave generating an electromotive force in the aerial, sufficient to break through the thin film of oil.

When the wave stops, the circuit is again interrupted automatically.

This device is used without a relay to actuate directly a syphon recorder as used in submarine telegraphy. The working battery employed with it must only have an electromotive force of about a tenth of a volt. It may be used also with a telephone in circuit and can, therefore, be employed either for telegraphic or telephonic reception.[45]

[Ill.u.s.tration: FIG. 17.--ITALIAN NAVY SELF-RESTORING k.u.mASCOPE. C, carbon plug; I, iron plug; M, mercury globule; A, aerial; B, battery; T, telephone; S, adjusting screw.]

One of the most sensitive of these self-restoring k.u.mascopes is the carbon-steel-mercury coherer, the invention of which has been attributed to Castelli, a signalman in the Italian Navy,[46] but also stated on good authority to have been the invention of officers in the Royal Italian Navy, and has, therefore, been called the Italian Navy coherer.[47] This instrument has been arranged in several forms, but in the simplest of these it consists of a gla.s.s tube, having in it a plug of iron and a plug of arc-lamp carbon, or two plugs of iron with a plug of carbon between them. The plugs of iron, or of iron and carbon, are separated by an exceedingly small globule of mercury, the size of which should be between one and a-half and three millimetres.

The plugs closing the tube must be capable of movement, one of them by means of a screw, as shown in the diagram (Fig. 17), taken from a patent specification communicated to Mr. Marconi by the Marchese Luigi Solari, of the Royal Italian Navy. One of the plugs of this tube is connected to the aerial and the other to the earth, and they are also connected through another circuit composed of a single dry cell and a telephone. The arrangement then forms an extremely sensitive detector of electric waves or of small electromotive forces, or, if a wave falls on the aerial, the electromotive force at once improves the contact between the mercury and the plugs, and therefore causes a sudden increase in the current through the telephone, giving rise to a sound; but when the wave ceases, or the electromotive force is withdrawn, the resistance falls back again to its origin value, and the arrangement is, therefore, self-acting, requiring no tapping or other device for restoring it to receptivity.

A very ingenious form of combined telephone and coherer has been devised by T. Tommasina.[48] In this instrument the diaphragm of an ordinary Bell telephone carries upon it a very small carbon or metallic coherer. This coherer is connected in between the aerial and the earth, and is also in circuit with a battery and the electromagnet of a telegraphic relay. When this relay operates it closes the circuit of another battery which is placed in series with the telephone coil.

The moment the current pa.s.ses through the telephone coil it attracts, and therefore vibrates, the diaphragm and shakes up the metallic filings. If an observer, therefore, places the telephone to his ear, he hears a sound corresponding to every train of waves incident upon the aerial. With this arrangement, one can obtain two different kinds of results, according to the nature of the cohering powder placed in the cavity in the diaphragm. First, if the powder consists of a non-magnetic metal, gold, silver, platinum or the like, the receiver will be very sensitive; and at the same time the current pa.s.sing through it when it is cohered will be sufficient to work a sensive recording apparatus in series with the telephone coil. Secondly, if the metallic powder placed in the cavity is a magnetic metal, the receiver will be somewhat less sensitive, but will work with more precision, because of the magnetic action of the magnet of the telephone upon the cohering powder. If no recording apparatus is used, the observer must write down the signals as heard in the telephone, since corresponding to a short spark at the transmitting station, a single tick or short sound is heard at the telephone, and corresponding to a series of rapidly successive sparks, a more prolonged sound is heard in the telephone. These two sounds, as already explained, const.i.tute the dot and the dash of the Morse signals.

We may, in the next place, refer to that form of k.u.mascope in which the action of the wave or of electromotive force is not to decrease the resistance of a contact, but to increase that of an imperfect contact. As already mentioned, Professor Branly discovered long ago that peroxide of lead acts in an opposite manner to metallic filings, in that when placed in a Branly tube it increases its resistance under the action of an electric spark, instead of decreasing it. Again, Professor Bose has found that fragments of metallic pota.s.sium in kerosene oil behave in a similar manner, and that certain varieties of silver, antimony and of a.r.s.enic, and a few other metals, have a similar property. Branly tubes, therefore, made with these materials, or any arrangements which act in a similar manner, have been called "anti-coherers." The most interesting arragement which has been called by this name is that of Schafer.[49] Schafer's k.u.mascope is made in the following manner: A very thin film of silver is deposited upon gla.s.s, and a strip of this silver is scratched across with a diamond, making a fine transverse cut or gap. If the resistance of this divided strip of silver is measured, it will be found not to be infinite, but may have a resistance as low as forty or fifty ohms if the strip is thirty millimetres wide. On examining the cut in the strip with a microscope, it will be found that the edges are ragged and that there are little particles of silver lying about in the gap. If, then, an electromotive force of three volts or more is put on the two separated parts of the strip, these little particles of silver fly to and fro like the pith b.a.l.l.s in a familiar electrical experiment, and they convey electricity across from side to side. Hence a current pa.s.ses having a magnitude of a few milliamperes. If, however, the strip is employed as a k.u.mascope and connected at one end to the earth and at the other end to an aerial, when electric waves fall upon the aerial, the electrical oscillations thereby excited seem to have the property of stopping this dance of silver particles and the resistance of the gap is increased several times, but falls again when the wave ceases.

If, therefore, a telephone and battery are connected between two portions of the strip, the variation of this battery current will affect the telephone in accordance with the waves which fall upon the aerial, and the arrangement becomes therefore a wave-detecting device.

It is said to have been used in wireless telegraph experiments in Germany up to a distance of ninety-five kilometres.

We must next direct attention to those wave-detecting devices which depend upon magnetisation of iron, and here we are able to record recent and most interesting developments. More than seventy years ago Joseph Henry, in the United States, noticed the effect of an electric spark at a distance upon magnetised needles.[50] Of recent times, the subject came back into notice through the researches of Professor E.

Rutherford,[51] who carried out at Cambridge, England, in 1896, a valuable series of experiments on this subject. He found that if a magnetised steel needle or a very small bundle of extremely thin iron wires is magnetised and placed in the interior of a small coil, the ends of which are connected to two long collecting wires, then an electric wave started from a Hertz oscillator at a distance causes an immediate demagnetisation of the iron. This demagnetisation he detected by means of the movement of the needle of a magnetometer placed near one end of the iron wire. Although Rutherford's wave detector has been much used in scientific research, it was not, in the form in which he used it, a telegraphic instrument, and could not record alphabetic signals.

Not long ago Mr. Marconi invented, however, a telegraphic instrument based upon his discovery that the magnetic hysteresis of iron can be annulled by electric oscillations. In one form, Mr. Marconi's magnetic receiver is constructed as follows[52] (see Fig. 18): An endless band of thin iron wire composed of several iron wires about No. 36 gauge, arranged in parallel, is made to move slowly round on two pulleys, like the driving belt of a machine. In one part of its path the wire pa.s.ses through a gla.s.s tube, on which are found two coils of wire, one a rather short, thick coil, and the other a very fine, long one. The fine, long coil is connected with a telephone, and the shorter coil is connected at one end to the earth and the other to the aerial. Two permanent horse-shoe magnets are placed as shown n Fig. 18, with their similar poles together, and, as the iron band pa.s.ses through their field, a certain length of it is magnetised, and owing to the hysteresis of the material, it retains this magnetism for a short time after it has pa.s.sed out of the centre of the field. If then an electric oscillation, coming down from the aerial, is pa.s.sed through the shorter coil, it changes the position of the magnetised portion of the iron and, so to speak, brings the magnetised portion of iron back into the position it would have occupied if the iron had had no hysteresis. This action, by varying the magnetic flux through the secondary coil, creates in it an electromotive force which causes a sound to be heard in the telephone connected to it. If at a distant place a single wave or train of waves is started and received by the aerial, this will express itself by making an audible tick in the telephone, and if several groups of closely adjacent wave trains are sent, these will indicate themselves by producing a rapid series of ticks in the telephone, heard as a short continuous noise and taken as equivalent to the Morse _dash_.

[Ill.u.s.tration: FIG. 18.--MARCONI MAGNETIC RECEIVER. W_{1}W_{2}, wheels; I, iron wire band; P, primary coil; S, secondary coil; T, telephone; A, aerial; E, earthplate.]

It was by means of this remarkably ingenious instrument that Mr.

Marconi was able, in the summer of 1902, to detect the waves sent out from Poldhu on the coast of Cornwall, and receive messages as far as Cronstadt in the Baltic, in one direction, and as far as Spezzia in the Mediterranean in another direction, and also to receive messages across the Atlantic from the power stations situated in Glace Bay, Nova Scotia, and from one at Cape Cod in Ma.s.sachusetts, U.S.A., in December, 1902.

There can be no question that this magnetic detector of Mr. Marconi's, used in connection with a good telephone and an acute human ear, is the most sensitive device yet invented for the detection of electric waves and their utilisation in telegraphy without continuous wires. It is marvellously simple, ingenious and yet effective, as a Hertzian wave telegraphic receiver.

Whilst on the subject of magnetic wave detectors, the author may describe experiments that he has been recently making to construct a Hertzian wave detector on the Rutherford principle, which shall be strictly quant.i.tative. All the receivers of the coherer type and electrolytic type give no indications that are at all proportional to the energy of the incident wave. Their indications are more or less accidental and depend upon the manner in which the receiver was last left. There is a great need for a quant.i.tative wave detector, the indications of which shall give us a measure of the energy of the arriving wave. It is only by the possession of such an instrument that we can hope to study properly the sending powers of various transmitters or the efficiency of different forms of aerial or devices by which the wave is produced. This magnetic receiver is constructed as follows:

A coil of fine wire is constructed in sections like the secondary coil of an induction coil, and in the instrument already made, this coil contains thirty or forty thousand turns of wire. In the interior of this coil are placed a number of little bundles of fine iron wire wound round with two coils, a fine wire coil which is a magnetising coil, and a thicker wire coil which is a demagnetising coil. These sets of coils are joined up, respectively, in series or in parallel.

Then, a.s.sociated with this form of induction coil is a commutator of a peculiar kind, which performs the following functions when a battery is connected to it and when it is made to revolve by a motor or by clockwork. First, during part of the revolution, the commutator closes the battery circuit and magnetises the iron cores, and whilst this is taking place the secondary circuit of the induction coil is short-circuited and the galvanometer is disconnected from it.

Secondly, the magnetising current is stopped, and soon after that the secondary coil is unshort-circuited and connected to the galvanometer, and remains in this condition during the remainder of the revolution.

This cycle of operations is repeated at every revolution. If then an electrical oscillation is sent into the demagnetising coils, and if it continues longer than one revolution of the commutator, it will demagnetise the iron core during that period of time in which the battery is disconnected and the galvanometer connected. The demagnetisation of the iron which ensues produces an electromotive force in the secondary coil and causes a deflection of the galvanometer, and this deflection will continue and remain steady if the oscillation persists. Moreover, since this deflection is due to the pa.s.sage through the galvanometer of a rapid series of discharges, it is large when the oscillations continue for a long time and are powerful, and small when they continue for a short time or are weak.

We can, therefore, with this arrangement, receive on the galvanometer, just as on the mirror galvanometer used in submarine cable work, a dot or dash, and, moreover, the magnitude of these deflections is a measure of the energy of the wave.

It is probable that when this arrangement is perfected it will become exceedingly useful for making all kinds of tests and measurements in connection with Hertzian telegraphy, even if it is not sensitive enough to use as a long distance receiver.

Of late years a variety of wave-detecting devices have been brought forward which depend upon electrolysis. One of the best known of these is that by De Forest and Smythe.[53] In this arrangement, a tube contains two small electrodes like plugs, which may be made of tin, silver or nickel, or other metal. The ends of these plugs are flat and separated from each other by about one two-hundredth of an inch.

Sometimes the end of one of these plugs is made cup shaped and the cup or recess is filled with a ma.s.s of peroxide of lead and glycerine. In the interval between the electrodes is placed an electrolyzable mixture, which consists of glycerine or vaseline mixed with water or alcohol, and a small quant.i.ty of litharge and metallic filings. These metallic filings act as secondary electrodes. When a small electromotive force is applied between the terminals of the electrodes of this tube through a very high resistance of twenty or thirty thousand ohms, an exceedingly small current pa.s.ses through this mixture, and it causes an electrolytic action which results in the production of chains of metallic particles connecting the two electrodes together. If, in addition to this, one terminal or electrode of the arrangement is connected to an aerial wire and the other terminal to the earth, then on the arrival of an electric wave creating oscillations in the wire, these oscillations pa.s.s down into the electrolytic cell, where they break up the chains of metallic particles and thus interrupt the current pa.s.sing through the telephone quite suddenly, which is heard as a slight tick by an ear applied to it. As soon as the wave ceases, the chain of metallic particles is re-established, so that the appliance is always in a condition to be affected by a wave. It is said that this breaking up and reformation of the chains of metallic particles is so rapid that a short spark made at the transmitting station is heard as a tick in the telephone, but a rapid succession of oscillatory sparks is heard as a short continuous sound; hence the two signals necessary for alphabetical conversation can be transmitted.

Another receiver which has some resemblance to the above, although different in principle, is that of Neugschwender.[54] In this arrangement, which to a certain extent resembles the Schafer detector, a gla.s.s plate has upon it a deposit of silver in the form of a strip, which is cut across at one place, thus interrupting it. If the cut is breathed upon or placed in a moist atmosphere, a little dew is deposited upon the gla.s.s, which bridges over the cut in the metal and creates an electric continuity. Hence a small current can be pa.s.sed across the gap and through a telephone by one or two cells of a battery. If, however, an electric oscillation pa.s.ses across the gap on its way from an aerial to the earth, then the continuity of the liquid film is destroyed, and the current is interrupted and a sound created in the telephone.

The opinion has been expressed by Sir Oliver Lodge that in this case the interruption of the circuit which occurs is really due to the coalescence of minute water particles into larger drops, as when vapour is condensed into rain, and hence the continuity of the material is interrupted.

We must then make a brief reference to other k.u.mascopes which depend upon the heating power of an electrical oscillation, which it possesses in common with every other form of electric current.

Professor R. A. Fessenden[55] has constructed a very ingenious thermal receiver in the following manner: An extremely fine platinum wire, about 0003 of an inch in diameter, is embedded in the middle of a silver wire about one tenth of an inch in diameter, like the wick of a candle. This compound wire is then drawn down until the diameter of the silver wire is only 0002 of an inch, and hence the platinum wire in its interior, being reduced in the same ratio, will have been drawn to a diameter of 000006 of an inch. A short piece of this drawn wire is then bent into a loop and the ends fixed to wires. The tip of the loop is then immersed in nitric acid and dissolved in the silver, leaving an exquisitely fine platinum wire a few hundredths of an inch in length and having a resistance of about thirty ohms. This little loop is sealed into a gla.s.s bulb like a very small incandescent lamp, or it may be enclosed in a small silver bulb and the air may be exhausted. If an electrical oscillation is sent through this exceedingly fine platinum wire it heats it and rapidly increases its resistance. The electrical oscillations produced in an aerial are sent through a number of these loops arranged in parallel, and the loops are short-circuited by a telephone, joined in series with a source of very small electromotive force produced by shunting a single cell or opposing to one another two cells of nearly equal electromotive force.

Any variation of resistance of the little platinum loops due to the heat produced by the oscillations, by suddenly altering the current flowing through the telephone, will cause a sound to be heard in it.

The electrical oscillations when pa.s.sing through the loops are therefore detected by the heat which they generate in these exquisitely fine platinum wires.

Finally, one word must be said on the subject of electrodynamic receivers, due to the same inventor. An exceedingly small silver ring is suspended by a quartz fibre and has a mirror attached to it in the manner of a galvanometer. This ring is suspended between two coils joined in series, which are placed either in the circuit of the aerial or in the secondary circuit of the small air core transformer inserted between the aerial and the earth. When electrical oscillations travel down the aerial they induce other electrical oscillations in the silver ring, and if the ring is so placed that its normal position is with its plane inclined at an angle of forty-five degrees to the plane of the fixed coils, then the ring will be slightly deflected every time an oscillation occurs in the aerial.

Omitting further mention of the details of the k.u.mascopes in use and the receiving aerial, we must next proceed to consider the receiving arrangements taken as a whole.

In the original Marconi system, the sensitive tube or coherer was inserted between the bottom of the receiving aerial and the earth.[56]

Accordingly, when the incident electric wave strikes the receiving aerial and creates in it an oscillatory electromotive force, this last will, if of sufficient amplitude, cause the particles of the coherer to cohere and become conductive. This sudden change from a nearly perfect non-conductivity to a conductive condition is made to act as a switch or relay, closing or completing the circuit of a single cell, and so sending a current through an ordinary telegraphic relay, closing or completing the circuit of a single cell, which may in turn actuate another recording telegraphic instrument, such as a Morse printer. To prevent the oscillations from pa.s.sing into the relay circuit, small choking or inductance coils are inserted between the ends of the sensitive tube and the relay and cell and serve to confine the oscillations to the tube.

It has already been pointed out that in the transmitting aerial the amplitude at the potential vibrations increases from the bottom to the top, and when vibrating in its fundamental manner there is a potential node at the earth connection and a potential loop or antinode at the top. The same is true of the receiving aerial. Hence, if the k.u.mascope employed is a Branly metallic filings tube and is inserted near the base of the aerial, the difference of potential between its two ends will be small.

It has also been mentioned that a receiver of this type acts in virtue of electromotive force or potential difference, and hence the proper place to insert the coherer is not at the base of the aerial, but between the top of the aerial and the earth. This, however, could not be done by running up another wire from the earth, as that would amount to putting the coherer between the tops of two identical aerials, and between its ends there would be no difference of potential. Professor Slaby, in conjunction with Count von Arco, has given an ingenious solution of this problem. If we take two equal lengths of wire, bent at right angles, and connect the point of intersection with the earth, placing one of these wires vertically and the other horizontally, we then have an arrangement which responds to the impact of electric waves, and has electrical oscillations set up in it in such fashion that the common point of the two wires has a very small amplitude of potential, but the two extremities have equal and large variations. If, then, we insert a coherer tube between the earth and the outer extremity of the horizontal wire, it is influenced in the same manner as it would be by the potential variations at the top of the vertical wire. In other words, it is acted upon by a large difference of potential instead of a small one. It is not found necessary to stretch the horizontal wire out straight; it may be coiled into a spiral with open turns, and the slight decrease in capacity and increase in inductance resulting from this can be compensated by cutting off a short piece of it.

[Ill.u.s.tration: FIG. 19.--SLABY RECEIVER. A, aerial; E, earth plate; F, coherer; M, multiplier; C, condenser; R, relay; B, battery; E, earth plate.]

In this way we have an arrangement (see Fig. 19) in which the outer extremity of this open spiral experiences variations or potential which exactly correspond with those at the summit of the vertical aerial. The receiving arrangements are then completed as in Fig. 19, one end of the coherer being attached to the outer end of the spiral and the other end through a condenser to the earth, a relay and a voltaic cell being arranged as shown in the diagram. The mode of operation of this receiver is as follows: When the wave strikes the aerial it sets up in it electrical oscillations with a potential antinode at the summit, and at the same time a potential antinode is created at the outer end of the spiral attached near the base of the aerial, this spiral being called by Professor Slaby a _multiplicator_. As long as the coherer tube remains non-conductive, the local cell cannot send a current through the relay, but, as soon as the resistance is broken down by the impact of a wave, the local cell sends a current through the coherer tube which, pa.s.sing down to the earth through the base of the aerial and up through the earth connection to the condenser, completes its circuit through the relay. Many variations of this arrangement have been made by Slaby and Von Arco and by the Allgemeine Elektricitats Gesellschaft of Berlin.

In 1898, Mr. Marconi made a great advance in the construction of his receiving apparatus by the insertion of his "jigger" or oscillation transformer in the aerial receiving circuit.[57] In this arrangement, the primary coil of an air core transformer wound in a particular way is inserted between the receiving aerial and the earth, and the secondary circuit is cut in the middle and connected to the two surfaces of a condenser, these surfaces being also connected through the circuit of an ordinary telegraphic relay and a single cell (see Fig. 20). The ends of the secondary circuit of this oscillation transformer are also connected to the terminals of the coherer tube, and these again are short-circuited by a small condenser.

[Ill.u.s.tration: FIG. 20.--MARCONI RECEIVER. A, aerial; J, jigger; CC, condensers; F, filings tube; T, tapper; R, relay; B, battery; M, Morse printer.]

The operation of this receiver is as follows: The oscillations set up in the aerial pa.s.s through the primary circuit of the jigger, and these induce other oscillations in the secondary circuit; the electromotive force or difference of potential between the primary terminals being transformed up in any desired ratio. It is this exalted electromotive force which is made to act on the coherer tube, and, inasmuch as the jigger operates in virtue of a current pa.s.sing through its primary circuit and this current is at a maximum at the lower end of the aerial, the arrangement is exceedingly effective, because it, so to speak, converts current into voltage. At the lower end of the aerial, although the amplitude of the potential oscillations is a minimum, the amplitude of the current oscillations is a maximum, and the jigger transforms these large current oscillations into large potential oscillations, _provided it is constructed in the right manner_. We can also transform up or increase the amplitude of the small potential variations near the bottom of the aerial by employing the principle of resonance. Many devices of this kind, due to Professor Slaby and others, have been suggested and tried but the details are rather too technical to be fully described here.

It will be noticed that the receiving aerial may be arranged in one of two ways--it may be either earthed at the lower end or it may be insulated. It has been claimed that there is a great advantage in earthing the receiving aerial directly in that it eliminates atmospheric disturbances.

We shall allude to this point more particularly later on. Meanwhile it may be mentioned that the receiving arrangements, as a whole, const.i.tute a sensitive arrangement, as shown by Popoff, Tommasina and by all the large experience of Mr. Marconi himself for detecting changes in the electrical condition of the atmosphere, which are doubtless of the nature of electrical oscillations. On the other hand, the receiving arrangements may be perfectly insulated and some experimentalists have a.s.serted that by this method the greatest freedom is secured from atmospheric disturbances. Amongst the non-earthed arrangements the system invented by Professor F. Braun, of Stra.s.sburg, and worked by Messrs. Siemens, of Berlin, may be mentioned.[58]

[Ill.u.s.tration: FIG. 21.--BRAUN'S NON-EARTHED RECEIVER. I, induction coil; CC, condensers; S, spark gap; J, transmitting jigger; K, receiving jigger; F, filings tube; R, relay; B, battery.]

Professor Braun's arrangements are indicated in the diagram in Fig.

21. In this case an induction coil is used to create a discharge between two spark b.a.l.l.s, and to these two b.a.l.l.s are connected the two outer coatings of two condensers, the inner coatings of which are connected together through the primary coil of an air core transformer. The secondary coil of this transformer is connected to two extension wires forming a Hertz resonator, and the length of these wires is so adjusted with reference to the time period of the primary circuit that they resonate to it, the whole length from end to end of the secondary circuit being half a wave-length. The receiver, as shown in the diagram, consists of a pair of quarter wave-length receiving wires connected through two condensers, which are short-circuited by the primary coil of an oscillation transformer. The secondary circuit of this last oscillation transformer has two extension wires to it, turned in the same manner, to respond to the primary oscillator; and in the circuit of one of these extension wires is placed a coherer tube, short-circuited by a relay and a local battery.

It will thus be seen that there is an entire abolition of ground connection, which, Professor Braun claims, practically avoids all atmospheric disturbances.[59] The details of the receiving arrangement are as follows:--The coherer tube consists of an ebonite tube containing hard steel particles of a uniform size, placed in the adjustable s.p.a.ce between two polished steel electrodes. It is found that with this steel coherer, a small amount of magnetism in the particles increases its sensitiveness, and to obtain this, a ring magnet is employed in connection with a coherer tube. Receiving apparatus arranged on this system is said to have been used for telegraphing between Heligoland and Cuxhaven, a distance of thirty-six miles.