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A History of Wireless Telegraphy (2nd edition, revised), J. J. Fahie, 1901, pages 130-134:


    The last example of a wireless telegraph with which we have to deal in this part of our history is an arrangement devised by Prof. Rathenau of Berlin, with the assistance of Drs Rubens and W. Rathenau, and which was found to be practicable up to a distance of three miles in water.
    Reports of the experiments of Messrs Preece, Stevenson, and others in England having appeared in the technical journals on the Continent, Prof. Rathenau, at the request of the Berlin Electrical Society, undertook to make a thorough investigation of the subject de novo.
    After a careful study of the work of these electricians he felt convinced that the favourable results obtained in England, especially by Preece, were largely due to conduction. To verify this opinion he commenced a course of rigorous experimentation; and to prevent inductive effects entering into the calculation he decided to use ordinary battery currents, and in one direction only.
    The outcome of the inquiry was published in an article which he contributed to the Berlin 'Elektrotechnische Zeitschrift', 77 from which I make a few extracts. When acurrent is sent through two electrodes immersed in a conducting liquid, the electrical equilibrium between these electrodes is not effected in a straight line, but in lines which spread out in the manner shown in fig. 15. Now, if we place in the liquid medium an independent conductor of electricity, it will attract or condense upon its surface a certain number of these lines, which can be utilised for the excitation of a properly constructed receiving apparatus. The distance at which these electrical effects can be produced is found to depend upon two factors--the available current strength and the distance between the electrodes.
Fig. 15
    It was thought best to conduct the experiments on the lake Wannsee near Potsdam, on account of the facilities in the way of apparatus afforded by the proximity of an electric-light station. The arrangement is shown in fig. 15. AB is a battery of 25 cells, W a set of resistance coils (0 to 24 ohms), SU an interrupter driven by a motor, AM an ampère-meter, meter, VM a voltmeter, T a Morse key, EP EP two zinc plates immersed in the water, 500 yards apart, and connected by cable as shown. The receiving circuit comprises two zinc plates, EP1 and EP1, suspended by cable X from two boats, from 50 to 100 yards apart, and nearly three miles from the sending station; N N are telephones included in the circuit of X. For the purpose of transmitting signals, intermittent currents were sent from the battery, which, by depressing the key for long and short intervals, could be heard in the telephones as dashes and dots of the Morse code. Fig. 16
    The object was to establish experimentally the best relation between the various factors--i.e., the relation between the current strength in the primary circuit and the hearing distance for the telephones in the secondary circuit; the effect of various distances between the electrodes EP EP upon the clearness of the signals; the distance between EP1 EP1 which gave the most audible effect; and, finally, the effect of altering the shape and size of the plates.
    On account of the non-arrival of some apparatus specially designed for these tests, the average current strength sent through the water did not exceed three ampères with 150 intermissions or current impulses per second. Again, the water of the Wannsee containing but a very small admixture of mineral salts offered a high resistance, so that it was found necessary to use large plates of 15 square yards surface.
    With this arrangement no difficulty was encountered in the transmission of signals from the electric-light station to the boats anchored off the village of New Cladow--a distance, as has been said, of nearly three miles; and Prof. Rathenau was satisfied that, by a slight change in the construction of the ordinary telephone, signals could be sent over much greater distances.
    "Lord Rayleigh," he says, "has stated that the sensitiveness of the telephone for currents with 600 reversals per second is about 600 times greater than for currents having but 130 reversals per second, but in my experiments the number of impulses did not exceed 150 per second. To get the best possible result in this system of transmission, a telephone should be used having a carefully tuned metallic tongue in place of the ordinary iron disc. Then, knowing the number of current-breaks in the primary circuit, the tongue should be so tuned as to vibrate in unison with that number, thereby producing much more distinct signals.
    "I may point out that the resistance of the receiving circuit should be as small as possible. At first I found it difficult to produce a call in the distant receivers, but this apparently knotty problem may be solved by attaching a microphone to the membrane of the receiver, which, acting upon a relay in a local circuit, produces the call.
    "It does not seem necessary to point out that by the use of several current generators, each one producing a definite number of current impulses, a number of noninterfering messages may be sent through the water to distant telephones, each being constructed to respond to but one definite rate of vibration; or by means of one current generator a message may be sent (simultaneously) to several distant telephone receivers.
    "The usefulness of this method of transmission would be much increased if means can be found to produce a written message. On the suggestion of Dr Rubens an apparatus is now being constructed, generally on the plan of Dr Wien's optical telephone. It is expected that the use of this apparatus will enable us to transform the acoustical into optical signals, and to register these photographically."
    Fig. 16 shows the locality of these experiments. It will be noticed that a large sandbank intervenes between the stations, but without any appreciable effect on the results.
    Prof. Rathenau concludes a very interesting paper with the enumeration of the chief points to be observed for increasing the effective signalling distance
    "1. Great current strength in the primary circuit.
    "2. Increasing the distance between the primary electrodes.
    "3. Increasing the distance between the receiving electrodes.
    "4. Replacing the metallic diaphragm of the telephone receiver by a light tongue.
    "5. Which should be tuned to respond to a definite rate of vibration." 78

    77 Abstract in 'Scientific American Supplement,' January 26, 1895, which I follow in the text.
    78 Experiments, based on the same conductive principle, were tried in Austria about the same time, but with what success I cannot say, as the results, for military reasons, have not been published.
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