We have seen (supra) that the want of some form of wireless telegraphy was particularly felt at a very early date in India, where the rivers are many and wide, and where for various reasons cables are liable to frequent breakage, causing interruptions which are as likely as not to be of long duration, owing to the great rush of waters and the flooding of banks.
I have already given some account of Dr O'Shaughnessy's experiments in this direction. It is all too short, but, unfortunately, it is all that I have been able to gather.
About the year 1858 Mr Blissett, a superintendent in the Indian Telegraph Department, resumed the inquiry, and obtained a fair measure of success by employing land-lines of considerable length on each bank of the river. In 1876 Mr Schwendler, then electrician, made some trials across the Hooghly at Barrackpore, near Calcutta, which were continued at intervals by his successor, the late Mr W. P. Johnston.
On September 9, 1879, this gentleman tried the following arrangement for signalling across the water of a canal. Fig. 11 shows the connections:--
E = 10 Bunsen's cells joined in series;
R, a needle instrument having a resistance of 1 ohm; also a telephone having a resistance of 4·25 ohms;
W = a resistance of 1 ohm e = four Minotto cells joined parallel
This arrangement exactly balanced the natural current through the receiving instrument.
A, B, C, D were copper plates, 8 feet 8 inches by 4 feet 4 inches by 1-16th inch thick, buried on the banks of the canal. B was buried 15 yards distant from A, and D the same distance from C. All the plates were parallel to the canal. The resistance between A and B was 7·5 ohms, and that between C and D was the same. Under these conditions, both the needle instrument and the telephone gave distinct and readable signals.
After several days of experiment with another method (fig. 12), using a single bare 600 lb. per mile galvanised wire, the following results were obtained:--
E = 15 Bunsen's cells in series;
R, a polorised Siemens relay of 21 ohms resistance;
e = 4 Minottos joined parallel W = 10 ohms
Balanced the natural current.
The signals received were quite regular and safe; the tongue of a relay worked an ordinary sounder in local circuit, and no difficulty was experienced in balancing the natural current through the relay.
A trial with bare wire for a distance of one and a half mile was not successful. Indeed, as it appeared that in order to obtain signals the battery power must be increased as the square of the distance, the limit of signalling through a bare wire under water is very soon reached.
Subsequently, three miles of the same wire, but partially insulated by being passed through a mixture of pitch and tar, answered perfectly for the hour that the instruments were in circuit.
At various times during the year 1888 Mr Johnston carried out many experiments across canals and the river Hooghly, and as the result of these and other careful investigations he was led to the following conclusions:--
1. That up to one and a half mile it is perfectly easy to signal through a bare wire under water.
2. That for greater distances, judging from experiments, practical signalling is not possible.
In April 1889 Mr Johnston died, and the duties of electrician were entrusted to Mr Melhuish, who immediately took up the inquiry, and in the end produced some very considerable results, for which, I believe, the Government of India gave him the handsome honorarium of 5000 rupees.
The results of his investigations are embodied in a paper which was read before the Institution of Electrical Engineers on April 10, 1890. "Having studied," he says, "the recorded labours of my predecessor, and learnt that by pursuing the same lines it was hopeless to expect to be able to signal through a bare wire across a river that had a greater breadth than one and a half mile, I resolved to change the class of signalling apparatus and to continue the experiment. Discarding continuous steady currents and polarised receiving relays, I adopted Cardew's vibrating sounder, and the sequel will show how completely successful the change of instruments proved to be. I began from the beginning, and tried to signal across a water-way without a metallic conductor by laying down two earth-plates on each of its opposite banks. Readable signals having been exchanged, the distance separating each pair of plates was varied, with the view of ascertaining how close the plates might be brought together, the signals remaining still readable. Readable signals were exchanged when the distance separating the plates was equal to the breadth of the river, reading becoming more difficult as the plates were made to approach each other, and clearer and more distinct as the distance between the plates was made to exceed the breadth of the river. I learnt from these experiments that in order to obtain signals of sufficient distinctness for the practical purpose of transmitting messages, it would be necessary to construct a line on each bank of a river much longer than the breadth of the river; and as the rivers along the coasts in India are extremely wide, I became impressed with the impracticable character of such an undertaking, and decided to strike out a new line.
"This new line was the laying of two bare uninsulated iron wires across the water-way parallel to each other, and separated by a certain distance, the ends on each bank being looped together by means of an insulated conductor. Hence, though much of the circuit was laid under water, it was nevertheless a continuous metallic circuit. Beginning first with a complete square, by laying the wires as many yards apart as the river was wide, signals were instantly exchanged that were incomparably louder than those that were exchanged when the same area was bounded by four earth-plates. The length of each of the two wires under water was next gradually increased to 740 yards, and the distance separating them gradually diminished to 35 yards, the strength of the signals diminishing proportionately, and ceasing to be readable when the wires were further approached. The conclusion arrived at from these experiments was that, for the practical and useful purpose of signalling messages across a broad river, in the absence of an insulated cable, a complete metallic circuit was at least desirable. Acting on this conclusion, it was sought to apply it practically, and the following experiment was carried out: At a distance of fifteen miles west of Calcutta a cable is laid across the river Hooghly, which at this point is 900 yards wide. The iron guards of this cable were employed to form one of the metallic conductors, and at a distance of 450 yards down-stream a single wire, weighing 900 lb. per mile, was laid across the river to form the second metallic conductor, insulated land-lines having been run up to loop the two parallel conductors together. The experiment was quite a success, the signals being readable without difficulty.
"An experiment was next made on a defective cable across Channel Creek, at the mouth of the river Hooghly. This creek is crossed by two cables laid in the same trench; the length of each is 3000 yards, and one of them had been completely parted by a steamer's anchor. Several attempts were made to signal across by using the guards of one of the cables as a lead, and the guards of the other as a return wire, but the efforts proved unsuccessful owing to the too close proximity of the cables. For every crossing there is a certain minimum distance apart at which the cables must be laid, and if this minimum, which depends on the breadth of the river, be exceeded, an absolute short-circuit becomes established. But although it was not possible here to signal through the iron guards, the most perfect signals were passed through the two conductors when they were formed into a loop, notwithstanding the fact that the two ends of the broken conductor were exposed in the sea and were lying at a considerable distance apart. An experiment was now made in order to ascertain what chance there might be in the future of signalling across the two conductors, should an accident occur to the good cable. Accordingly, the conductor of the good cable was disconnected in the cable-house from the signalling apparatus and placed upon the ground, when the signals, though greatly diminished in volume, still continued to be distinctly readable. It may, therefore, be reasonably inferred that should the good cable suffer a similar fate to that of the defective cable, communication can, by means of Cardew's sounders, be kept up by looping the ruptured conductors until arrangements can be made for laying a new cable or repairing the defective ones.
"It will probably suffice if from the succeeding experiments that were made to test the efficiency of the vibrating sounder in the case of conductors breaking down at river crossings I select the following three, exhibiting as they do progressive evidence of the value of this signalling instrument, and culminating in establishing it beyond dispute as one that can be relied on for carrying on independent communication through the iron guards of cables while the insulated copper conductors form parts of other circuits.
"Experiment No. 1.--The local line from the Central Office, Calcutta, to Garden Reach is about four miles in length, and at about midway the wire spans a small river. Vibrating sounders having been put in circuit at each end of this line, the wire where it crosses the river was taken down and laid along the bed of the water-way. Signals were loud and clear at both ends.
"From the success of this experiment it may be inferred that on any ordinary line, should the wire from accidental causes come off the insulator and make earth by touching the bracket, standard, or ground, or should the wire break and both ends of it be lying on the ground or in a watercourse, communication could still be maintained by means of the vibrating sounders.
"Experiment No. 2.--The line wire which connects the town of Chandernagore with Barrackpore is about ten and a half miles long, 900 yards of which consist of a cable laid across the river Hooglily. Vibrating sounders having been joined up in the telegraph offices at Barrackpore and Chandernagore, the insulated conductor of the cable was thrown out of circuit, and the line wire on each side of the river was joined to the iron guards of the cable. Thus for a length of half a mile out of ten and a half miles the conductor was wholly under water, yet it was found quite feasible to transmit messages between the two offices.
"From the success of this experiment it may be reasonably inferred that in the case of certain cable crossings, where the rivers are not too wide, should the copper conductor of the cable make dead earth, or become insulated by parting, communication could still be kept up between the two offices on either side.
"Experiment No. 3.--The terminus of the Northern Bengal State Railway at Sara is separated from that of the Eastern Bengal State Railway at Damukdia by the river Ganges. The opposite banks of the river in this locality are connected by two independent cable crossings. The length of one of these crossings is one mile 610 yards, and of the other four miles. The distance which separates the two cable-houses on the Damukdia side is three miles 1584 yards, and on the Sara side the cable-houses are only one mile 211 yards apart, giving a mean lateral distance in alignment of two miles 880 yards. The two cable-houses on each bank of the river have an insulated connecting landline.
"The connecting land-lines having been joined to the iron guards of the cables, two vibrating sounders were placed in circuit, one on each side of the river, when signals so strong were transmitted across that it was not difficult to read them at a distance of 6 feet away from the receiving telephone.
"From the marked success of this experiment it may be inferred that at all river cable crossings where the cables are laid in separate alignments (and the farther apart the better), should the cables become interrupted, communication may still be maintained from bank to bank by using vibrating sounders, thus avoiding the delay, inconvenience, and cost of a boat service.
"It should also be remembered in the case of such a parallel cable crossing that, besides the circuits afforded by the copper conductors when these are in working order, there is always an additional local circuit available by means of the iron guards between the opposite cable-houses, and that this circuit could be used by means of the vibrating sounder as a talking circuit, in cases of necessity, without interrupting through working on either of the cables.
"It is desirable in circumstances similar to these to reduce all the resistance external to the actual connecting lines to as small a quantity as possible, and therefore, when messages are being transmitted, the telephone at the sending end should be removed from the circuit, as also should the vibrator from the receiving end. To effect this twofold purpose a special form of signalling key is requisite, and should be used. The action of this key, together with the complete set of connections for a parallel cable crossing, is shown in fig. 13." 70