Hertz Wave Telegraphy by the Marconi System

(From our London Correspondent.)

At last the truth about the “Marconi mysteries” has been revealed. In a lecture at the Royal Institution on June 4, Mr. W. H. Preece told us what the Marconi invention really is — and a great many other things as well which we knew before. The sensational articles in the lay magazines and in the daily papers ascribed to Marconi the discovery (or invention, as it was perhaps more appropriately termed) of a new kind of electrical radiations which had quite different properties to Hertz rays, which would penetrate anything from a brick wall or a few hundred yards of solid hill to iron plates a foot or so in thickness and blow up the powder magazine of an enemy’s ironclad, and do several other truly remarkable and characteristic feats. Marconi has actually devised a very sensitive and seemingly reliable form of coherer, which can detect Hertz waves generated by a Righi radiator several miles distant.

Marconi no doubt deserves great commendation for having worked out an ingenious and efficient apparatus, and having brought us a step nearer to the practical utilization of Hertz waves; but the fact of his abstaining from stating in the interviews and articles that have been published that his work has been nothing more than modifications in the details of apparatus devised by his more illustrious predecessors, has entirely prevented him from receiving that genuine tribute of praise, which almost amounts to veneration, usually accorded so freely to successful workers in the realms of science.

The Marconi coherer consists of a small glass tube four centimetres long, into which two silver pole-pieces are inserted, separated from each other by about half a millimetre. This thin space is filled with a mixture of nickel and silver filings — 96 per cent. of nickel, 4 per cent. of silver and a trace of mercury. The tube is exhausted to a vacuum of 4 mm. and sealed. It forms part of a circuit containing a battery cell and a sensitive telegraph relay. Normally the resistance across the powder gap of the receiver is several thousands of ohms, but when the Hertz wave falls on it, it is reduced to about 5 ohms.

Mr. Preece did not say whether the increased sensitiveness of the relay was chiefly due to the composition or the finely divided state of the metal filings, or to the use of a vacuum. In other respects the coherer does not differ very much from that devised by Lodge some four years ago; the principle was first shown by M. E. Branly in 1890. The main defect in the Lodge coherer was that after receiving a wave it did not “decohere” without being shaken or tapped. Marconi gets over this difficulty by putting in the “local” circuit of the relay a small electrically worked hammer, so that on the reception of each signal the glass tube is lightly tapped. The sound made by this little hammer upon the glass is sufficient to read Morse signals from, but, of course, a Morse ink writer or any other instrument can be put in the local circuit.

The transmitter is, as already stated, Prof. Righi’s form of Hertz radiator. Two spheres of solid brass, 4 inches in diameter, are fixed in an oil-tight case of insulating material, filled with vaseline oil, so that a hemisphere of each is exposed, and the other hemisphere is immersed in the oil bath. The oil seems to maintain the surfaces of the spheres clean, thus obviating the frequent polishing required in the original apparatus of Hertz. Mr. Preece also stated that it impresses on the waves excited by the sparks between the spheres a uniform and constant form. Two smaller spheres are fixed close to the opposite sides of the two larger spheres, and these smaller ones are connected to the secondary of the induction coil. Morse signals are made by a key in the primary circuit of the coil. The waves produced by this apparatus are very short, generally about 120 cm. long. With a 6-inch spark coil, signals were transmitted to a distance of four miles; for greater distances sparks up to 20 inches were used. Mr. Preece has succeeded in transmitting signals over a distance of eight miles across the Bristol Channel with the Marconi apparatus.

For shorter distances the poles of the coherer are connected to two “wings” of copper, each in series with a small retardation coil. When signalling over longer distances the wings are removed, one pole of the coherer is connected to earth, and the other extended to the top of a mast or by a kite connected with aluminum wire to the coherer. It seems that during the experiments signals were actually received through hills. Mr. Preece explained this by a diagram in which the electric ray was depicted curving round the top of the hill in a very accommodating way. It would seem more probable, however, that, as the beam is naturally somewhat diffused, some rays may be reflected round the sides of the hill by other conducting obstacles, or perhaps the hill was actually not a good enough conductor to absorb the rays, and was to a certain extent transparent to them. Another “apparent anomaly” was that Marconi’s relay acts even when enclosed in a metal box. During the lecture Mr. Preece put the instrument in a galvanized iron dust bin, and yet it received the signals transmitted from another room. He also assured his audience that he had performed the same experiment in a hermetically sealed box. This would only prove, however, that the coherer is extremely sensitive and that the intensity of the waves transmitted is so great, that they are not entirely absorbed by the fraction of an inch of metal.

Besides intending to employ this system of telegraphy to signal to lightships and lighthouses from the shore, Mr. Preece intends to use it for a regular public telegraph service to the island of Sark, in the English Channel, as this island is at present not connected to any submarine cable.

When Marconi’s patents have been published an interesting question will be whether they have any commercial value. We believe that the use of a vacuum in a coherer has been proposed and even used before. The combination of a coherer with a relay is no new idea, and Lodge proposed in 1894 the employment of an electric vibrator to act as what Mr. Preece calls a decoherer. Moreover, without wishing to lessen the credit due to Marconi, we cannot but think that had the British Telegraph Administration given one of the great English physicists like experimental facilities, the same, or even better, results would have been obtained.