Telegraphy Across Space - Part 1/2

* Paper read before the Society of Arts, March 30th, 1898.

There is no such thing as wireless telegraphy. True, one can send signals for a distance of a yard or two without any wires, but in all the recent successful attempts to telegraph across space, whether by electric waves or by other means, wires are used. They do not, indeed, run from the sending station to the receiving station, but are used as base lines. For example, in the case of the longest distance yet reached in telegraphing by electric waves - 13 miles over open country - the maximum distance attained in the recent experiments of Prof. Slaby, the length of the wires used as base lines at each end was nearly 1,000ft. As will be seen, in every case, wires or their equivalent are used to serve either as base lines or as base areas in the transmission.

Setting aside the medieval myth of a sympathetic magnetic telegraph with two mere compass needles to point to letters ranged around a dial, there are three generic methods by which it has been found possible to signal across space without any directly communicating wire or cable. These may be conveniently classified as follows: (1) conduction methods; (2) induction methods; (3) wave methods.

I. - Conduction Methods

These methods depend upon the use of water or earth as a means of conducting a fraction of the electric current from the base line at the sending end to the base line at the receiving end.

From the earliest days of telegraphy it has been a familiar fact that either earth or water might serve as a return circuit for an electric current, and, under certain circumstances, that signals could be sent even with an absolute gap in the metallic line, if there were provided by means of earth or water a sufficiently good path to enable current in adequate amount to be received beyond the gap in the line. This method has sometimes been called the leakage method, since it depends upon the circumstance that electric currents flowing in a conducting medium, such as water or moist earth, do not flow exclusively or even mainly along the path of least resistance, but spread out, some flowing along paths of greater resistance. If current enters a conducting stratum at any point by a single electrode, A, and leaves it at some other point by another suitable electrode, B, some of the current will certainly flow straight from A to B; yet the greater part will not so flow, but will stream around from A to B in long curving paths. If, then, two other electrodes, C and D, are inserted in one of these stream paths at a distance from A and B, some of the current - perhaps only a small percentage of it - may be picked up by a metallic line joining C to D. Hence it is possible, using A B as a sending base line, to signal to C D as a receiving base line at a distant place. The only limits to this method of telegraphy across space are (1) the strength of the original currents used in the sending base line, A B; (2) the sensitiveness of the apparatus used in the receiving base line, C D; (3) the ratio between the space distance from A B to C D and the lengths of the two base lines. This system of telegraphing across space has been proposed at various times. It has been used by Mr. Preece in several of those many experiments which he has made from time to time, and which will entitle him to be regarded as one of the foremost pioneers in this entire branch of telegraphic enterprise.

Morse himself - as recorded in Vail’s early work on telegraphy - worked at this subject, and made experiments in 1842 on the Susquehanna river, about a mile wide. He engaged Prof. Gale to investigate the best conditions, and came to the conclusion that the base lines should be three times as long as the distance to be crossed. Mr. Dering, an English telegraph engineer, and Mr. Lindsay, of Dundee, have also worked in this direction.

After the introduction in 1877-78 of the Bell telephone it was found that the extraordinary sensitiveness of that instrument furnished a new means of picking up currents that would otherwise be too feeble to produce intelligible signals. The importance of this circumstance in extending the possibilities of distance telegraphy was not lost upon Mr. Preece. In 1882 he conducted a series of researches upon the establishment of telegraphic communication between the Isle of Wight and the Hampshire coast without any connecting cable across the Solent. An account of these experiments will be found in the report of the British Association for that year. Large metal plates to serve as electrodes were immersed in the sea at the ends of the two base lines. On the Hampshire coast the base line extended from Portsmouth through Southampton to Hurst Castle, a length of 20 miles. On the island the base line extended from Ryde through Newport to Sconce Point, and was about 16 miles long. From Portsmouth to Ryde the breadth of the sea is six miles, while Hurst Castle is only about a mile from Sconce Point. Hence in this case the length of the base lines considerably exceeded the average distance to be crossed. With this arrangement signals were passed in dot and dash which could be read on the Morse system with ease; but telephonic speech was not feasible. After many other experiments to be mentioned under the next heading, Mr. Preece established communication in the winter of 1893-4 across the Kilbrannen Sound between the Isle of Arran and Kintyre, a distance of over four miles. He also maintained telephonic speech across Loch Ness, a distance of 1¼ miles.

In the experiments from Arran to Kintyre, parallel wires about three miles long were used as base lines along the coast, while in some of the experiments two other base lines were used, being insulated wires laid along each coast at a height of about 500ft. above sea-level. A detailed account of these experiments will be found in the report of the British Association for 1894.

A year earlier Mr. Preece had made some striking experiments in the Bristol Channel between Lavernock Point on the South Wales coast and the islands of the Flat Holm and the Steep Holm, the distances of which are respectively 3·1 and 5·35 miles. His base line on the shore at Lavernock Point was a pair of copper wires weighing 400lb. per mile, suspended on poles for a length of 1,267 yards, their circuit being completed through earth. An alternating current was sent into this base line by an alternator worked by a 2-h.p. steam-engine, the voltage being 150 volts, the frequency 192 periods per second, and the current (maximum) 15 amperes. These alternations were broken up into dots and dashes by use of a Morse key. The signals were read on a pair of receiving telephones inserted in the distant base line, which in each case ran across the island and dipped into the sea. The length of these is not stated. Mr. Preece received messages easily over the three miles separating the mainland from the Flat Holm. But at the Steep Holm, 5·35 miles away, though the signals were feebly perceptible, telegraphic conversation was impracticable, as the sound could not be differentiated into dots and dashes. Mr. Preece came to the conclusion that with two base lines, each 10 miles long, he could with ease signal across a distance of 10 miles.

Prof. Trowbridge, of Harvard, has also investigated the possibility of transmitting signals through the earth by conduction, using a rapidly interrupted primary current and a telephonic receiving apparatus.

Many experiments have been made under accidental circumstances, all tending to prove the possibility of this mode of transmitting signals through the earth itself. The instruments in Greenwich Observatory are affected by the stray currents that escape into the earth from the badly-insulated return circuit of the City and South London Electric Railway, 4½ miles away. Another example is afforded by an accident which occurred some 10 years since at the Ferranti electric lighting station at Deptford, when one night one of the dynamos by some derangement became connected to earth. The whole of the railway telegraphs in the signal boxes of the railways in South London were temporarily put of order and rendered inoperative, while the currents flowing in the earth were perceived in the telegraph instruments so far northwards as Leicester, and so far south as in Paris. If this could occur as a mere accident, it is obvious that with properly-thought-out arrangements signals could easily be sent from one part of the globe to another by conduction through earth or water.

Most striking of all the cases of distance signalling by conductive methods is that presented by the transmission of signals over nearly three miles, which was carried out in 1894 by Dr. W. Rathenau, Mr. E. Rathenau, and Prof. Rubens. They selected as a suitable place for operations the open water of the Wannsee, which opens into the Havel, near Potsdam. Here at the south end, near the Friedrich-Wilhelmsbrücke, they immersed two metal electrodes, each having about 15 square metres of surface, at the two ends of a base line about 550ft. long. With 75 accumulators and a rotating interruptor giving about 150 currents per second, and a Morse key, they injected signals into the base line. At a distance of 4½ km., or nearly three miles across the water, near the shore at Neu Cladow, they set up the secondary base line, having electrodes of about four square metres each. These were hung in the water from two boats between which the connecting line - about 330ft. long - was stretched. In this line was inserted a telephone receiver of usual pattern. The current used was about three amperes, and there was not the slightest difficulty in hearing the dot-and-dash messages. Several situations for the receiving base line were tried, and it appeared that the interposition of a large sandbank between the two stations made very little difference.

II. - Induction Methods

Induction methods are of two varieties. An electric charge upon a conductor may induce another electric charge upon another conductor at a distance by influence, or electrostatic induction. An electric current in a wire, during such time as it is increasing or diminishing, may induce another electric current in another wire in its neighbourhood by electromagnetic induction.

So far as I am aware, the only case in which electrostatic induction has been used in electric signalling is that of telegraphing (or telephoning) to trains in motion, as suggested about 13 years ago by Mr. Wiley Smith, of Kansas City. If a wire suspended over a train is electrified, either positively or negatively, charges are induced upon the metallic roofs of the cars, and if these are suitably connected to instruments on board the train, signals may be exchanged between train and wire without any metallic connection between the two. This suggestion was further developed, about the year 1886, by Mr. Phelps, and by Messrs. Gilliland and Edison. Descriptions of their methods will be found in the American electrical journals of that date. The system was successful both for telegraphing and telephoning, and was, indeed, adopted for a time by the Lehigh Valley Railroad Company. But it has been abandoned for a very simple reason. One of the consolations of railway travelling is that one is free from being disturbed by telegraph or telephone. No one on board an express wants to telegraph or be telegraphed to.

Electromagnetic induction has played so important a part in distance telegraphy that it must receive a more extended notice. Very early after the introduction of the commercial telephone, troubles arose from the exceeding sensitiveness of the instrument. Conversations in one line were overheard in another, while the ear was disturbed by an incessant buzz or rattle from the interference of stray currents from neighbouring telegraph. All these were at first attributed to induction - that is to say, to the electromagnetic influence of the currents in one line upon the neighbouring line. No doubt in some cases this is a cause, but unquestionably in many of the cases the disturbance was due not to induction at all, but either to leakage of currents across the surfaces of the insulating supports, over films of dirt or moisture, or else to leakage of currents from one line into the other through the earth-plates or earth connections. Unless circuits with metallic returns are used it is certain that the earth return will afford a means for stray currents to find their way into the telephone lines. Mr. Preece has narrated many cases in which telegraph or telephone messages that are being transmitted along some line have been heard, or rather overheard, in telephonic instruments in some totally disconnected and distant line. Many of these are due doubtless to stray currents through earth, but some are unquestionably due to true induction. A line or circuit absolutely insulated from any earth contact or earth return may yet act inductively. During the brief instant while the current in it is growing that current is setting up a magnetic field in the surrounding region, extending indefinitely but feebly into space. As the current dies away again this magnetic field also dies away. If in its growth or decrease this magnetic field encounters other wires it sets up E.M.F.’s in them, and thus originates disturbances. For the propagation of this effect from wire to wire no contact is needed. It is an effect that is dependent upon the properties of the intervening medium, and is proportional to its magnetic permeability. The ether of space itself - air, water, soil, and rock - are all of about equal permeability. Hence this kind of induction may be propagated from circuit to circuit whatever natural material intervenes. Mr. Preece has made repeated researches with a view of utilising this effect for the purpose of distance telegraphy. He has erected parallel base lines, sometimes in South Wales, sometimes near the mouth of the Dee, sometimes in Scotland. He has laid out, flat on the ground, great squares of insulated wire to test the inductive transmission from one area to another. On Newcastle town moor, and on the sands at Penarth, he has thus operated. It is not always easy in his experiments, particularly in those where earth connections were used, to be certain how much of the effect was due to true induction and how much to earth conduction. But in some of the cases there can be no doubt whatever. An excellent résumé of his work was given by him at the Chicago Congress in 1893. In this he describes how in one series of experiments he laid out on a level plain two quarter-mile squares of copper wire insulated with guttapercha, the distances between the two nearest sides of the two squares being also a quarter of a mile. In this case, using rapidly-interrupted or vibratory currents, and a Morse key to break them up into Morse signals, and applying in the other circuit a receiving telephone, conversation in the Morse code could be held readily between the two operators. This arrangement precluded all idea of earth conduction. In effect, Mr. Preece was working with a strange species of transformer, of which his two squares constituted respectively the “primary” and the “secondary,” the “core” of the transformer being in this case partly of earth and partly of air. Mr. A. W. Heaviside has described an analogous case in which, wishing to establish telephonic communication to the bottom of a colliery in the North of England, he arranged a circuit in a triangular form along galleries about 2¼ miles in total length at a depth of 60 fathoms. On the surface of the colliery another circuit was laid out in triangular lines of equal size over and parallel to the underground line. Here, again, telephonic speech was perfectly clear by induction from line to line; or rather, in this case, from area to area. Each area enclosed something like 700,000 square yards, an ample base area when the distance to be penetrated was but 120 yards.

Earlier than the date of either of these experiments, the late Mr. Willoughby Smith had shown how, using two coiled circuits of wire at a distance of some yards apart, telephonic messages could be sent across air, or even through walls and floors.

The greatest distance to which Mr. Preece’s experiments upon telegraph lines have been carried is 40 miles - namely, between the telegraph lines that run across the Scottish border by the east and west coasts respectively. Sounds produced in the Newcastle and Jedburgh line were distinctly heard on the parallel line at Gretna, though there was no line connecting the two places. Here, however, since both lines used earth returns, it is probable that most of the effect was due to conduction, not to true induction.

Instruments which operate by means of alternating currents of high frequency, like Mr. Langdon-Davies’s phonophore, are peculiarly liable to set up disturbance in other circuits. A single phonophore circuit can be heard in lines 100 miles away. When this first came to my notice it impressed me greatly, and, coupled in my mind with the Ferranti incident mentioned above, caused me to offer to one of my financial friends in the City, some eight years ago, to undertake seriously to establish telegraphic communication with the Cape, provided £10,000 were forthcoming to establish the necessary basal circuits in the two countries, and the instruments for creating the currents. My offer was deemed too visionary for acceptance. The thing, however, is quite feasible. The one necessary thing is the adequate base lines or areas. All the rest is detail.

(Continued in Telegraphing Across Space - Part 2/2.)