Marconi Telegraphy

In July, experiments with this system were made at Spezzia by a Commission of the Royal Italian Fleet, assisted by the inventor. The results, of which the following is an abstract, are reported in the Rev. Marittima :—

A transmitter, and a receiver for controlling purposes, were set up on land; a second receiver being placed on board ship. In one experiment the ship lay at anchor, in another the ship steamed away from, or approached, the telegraphing station on shore. The results obtained were:—1. Under favourable atmospheric conditions, i.e., in the absence of electric strain in the air, messages were successfully transmitted from land to the ship in motion, over a distance of 8.9 nautical miles (10.5 miles). 2. The presence of electric strain in the atmosphere made the reading of messages by the Marconi apparatus impossible. 3. It was found, also, that the intervention of mountains, islands, or headlands between the transmitter and the receiver made the reading of the messages impossible. 4. The intervention of masts or chimneys, or the like, was also found to considerably reduce the distance at which messages could be distinctly read; for example, if the receiver was fixed abaft the ship, and the ship was steaming directly toward the sending station on shore, the distinctness of the messages was considerably reduced.

The account given by Mr. Marconi, in an interview published by the Daily Chronicle, is somewhat more sanguine.

He says:—

I sent messages from the Arsenal of San Bartolomeo and the San Martino, an armour-clad fighting ship, completely armed and cruising in the open sea 12 miles distant from the fortress. My receiver was hidden in the cabin, under the guns, under the engines, in fact, anywhere on board, so long as the vertical wire conductor remained exposed. No matter where the receiver was, it "ticked" out the message in due course. I used conductors 90 feet high to procure this result, and I am now preparing a receiver to take a message from a distance of 40 miles. The fact of the message being sent to a vessel which was practically one mass of metal made no difference. In Rome, to communicate to houses I had a conductor 3 feet high. For the transmission of signals across the Bristol Channel it was 100 feet high, but it is difficult to say off hand how far the question of distance is determined by the height of the conductor. You see I got signals to a greater distance at Spezia with a conductor 10 feet less in height. The induction coil used in Italy was much less powerful, as the signals were obtained by a 6-inch spark worked by a small portable battery of eight volts. This disproves the statement of certain scientists who have declared that my results have only been secured by the employment of a large amount of battery power.

In the Chronicle interview Mr. Marconi defends his inven- tion against the charges of want of novelty which have very freely been brought against it. He claims that he has greatly improved the Branly coherer, and invented for it the electro-magnetic tapper, a claim which has been questioned by Lodge. But we think there can be no doubt that Marconi is the true and first inventor of the elevated electrodes on the receiver and transmitter, and this detail appears to have contributed more to extend the possible distance of telegraphy by electric waves than anything that has been discovered since the time of Hertz and Branly. In the interview above referred to, Mr. Marconi insists on this point, saying:—

I think, too, that my invention deserves all the protection it has secured, for what I have really discovered is this, that by bringing one pole of the transmitter and one pole of the receiver in contact with the earth, and joining the other poles of the instruments to vertical conductors of suitable height, I can send a message 12 miles with an amount of battery power that would not obtain any effect at all, even at 100 yards, if used by the Branly, Lodge, or Righi apparatus.

No doubt the Law Courts will some day be asked to settle the differences between the rival claimants for the invention of wireless telegraphy. In the meantime the heated discussions on the subject which are appearing in some of our contemporaries appear to be futile.

M. Del Proposto gives the following mathematical theory of the working of the Marconi apparatus. Since the trans- mitter and receiver in the Marconi system are connected by induction, the efficiency and range of the apparatus can be determined by a consideration of the coefficient of mutual induction.

Cæteris paribus the current induced in the receiver is proportional to this coefficient, which is given by the expression

$$ M = \iint \frac{ds \, ds^1 \cos \epsilon}{r} $$ (1)

in which ds and ds¹ are elements of the two circuits, ε the angle between them, and r their distance apart. In the Marconi apparatus with vertical wires (taken as parallel) ε = 0, r = constant, the expression becomes

$$ M = {h^2 \over r} $$ (2)

h being the height of the wires. Equation (1) is reduced to equation (2) if, at any instant, the current has the same sign at every part of each apparatus, an admission which cannot, however, be made a priori. According to (2) the range of an apparatus of given sensibility will be proportional to the square of the height of the vertical wires. But it must not be forgotten that in increasing the length of the vertical wires, we increase their self-induction and their capacity. This, in accordance with the formula

$$ T = \frac{\pi \sqrt{c L}}{v} $$

(v = the velocity of light, T = duration of an oscillation) will reduce the frequency of the oscillations. The decrease of the range due to diminished frequency may be compensated by increasing the original current, i.e., the energy of the discharge.

The electromagnetic force is horizontal if the wires of the apparatus are vertical; consequently the transmission by the Marconi apparatus will not be disturbed by horizontal conductors, such as telegraph and telephone wires. But vertical conductors may cause serious disturbances in the transmission; Hertz indeed has shown that a frame of parallel wires suitably arranged in relation to the waves may stop them. If two receivers are placed at considerably different distances, the nearest receiver may absorb so much of the energy of the electric waves that it will be impossible to produce any effect on the more distant instrument. The electromagnetic waves are propagated in every direction, and if these waves encounter conductors capable of acting as receivers (and all conductors can do so to a certain extent), the transmission between the two instruments will be seriously interfered with.

Reflectors are not likely to be successfully used with the Marconi apparatus. Hertz found that reflectors were useless with waves 7 m. in length, and the Marconi waves are much longer than this. Before reflectors can be successfully used, it will be necessary to reduce very considerably the length of the waves, but these reflectors will have to be very large, comparable in dimensions, in fact, with the vertical wire.

The propagation in every direction is the principal objection to the Marconi system. Multiple reception of messages may, to a certain extent, be obviated by tuning, but it would always be easy to bring any receiver into tune by trial.