Latest / History of the Universe Physics & Electricity Vol. 3 / 031 - Electricity - Chapter 11 - Wireless Telegraphy Part 1

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031 - Electricity - Chapter 11 - Wireless Telegraphy Part 1 · History of the Universe Physics & Electricity Vol. 3

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  Speaker 1: Section thirty one of the Science History of the Universe,
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  Speaker 1: Volume three. This is a LibriVox recording. All LibriVox recordings
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  Speaker 1: are in the public domain. For more information or to volunteer,
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  Speaker 1: please visit LibriVox dot org. Recording by Emelia Chesley, The
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  Speaker 1: Science History of the Universe, Volume three, edited by Francis
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  Speaker 1: rold Wheeler. Electricity, Chapter eleven Wireless Telegraphy, Part one. Professor
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  Speaker 1: Henry of Princeton University was the first to show the
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  Speaker 1: oscillatory character of the discharge of a Leyden jar. This
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  Speaker 1: single loud spark, which to the eye seems to pass
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  Speaker 1: in one direction across the gap, is really a quick
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  Speaker 1: succession of current surges, first one way and then the other,
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  Speaker 1: and has its mechanical analogy in the pendulum at the outset.
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  Speaker 1: It may be well to analyze this spark discharge, as
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  Speaker 1: it is still the most mos prominent means of radiating
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  Speaker 1: the electric waves used to transmit signals. Suppose this pendulum
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  Speaker 1: analogy of the spark discharge between two spheres be taken,
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  Speaker 1: and the similarity of the two actions noted. If a
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  Speaker 1: heavy pendulum be drawn back by means of a light fiber,
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  Speaker 1: it will finally strain the fiber to such an extent
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  Speaker 1: as to cause it to break. The pendulum, being suddenly released,
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  Speaker 1: gradually acquires motion, which is accelerated until its lowest position
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  Speaker 1: is reached, after which it is retarded just as it
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  Speaker 1: was accelerated, and stops at about the same height at
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  Speaker 1: which it started. The process is then repeated, but in
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  Speaker 1: the opposite direction. Now compare the action on the two
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  Speaker 1: charged spheres. The pressure in the dielectric surrounding the spheres
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  Speaker 1: is gradually raised until it suddenly gives way, there being
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  Speaker 1: a flow of current from the positive to the negative sphere,
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  Speaker 1: which current gradually increases and is greatest at the instant
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  Speaker 1: when bold dielectrics are at the same potential, just as
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  Speaker 1: the motion of the pendulum was greatest at its lowest position.
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  Speaker 1: The current then begins to decrease and finally stops when
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  Speaker 1: the dielectric is again strained to about the same potential
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  Speaker 1: it had at first, but reversed indirection. The current then
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  Speaker 1: starts back again, just as the pendulum again required motion. Now,
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  Speaker 1: let it be considered what goes on in the space
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  Speaker 1: surrounding this action. A current is always surrounded by a
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  Speaker 1: field of magnetic force, which field is proportional to the
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  Speaker 1: strength of the current. As this current between the two
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  Speaker 1: spheres increases, there is therefore sent out an increasing magnetic
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  Speaker 1: field which is radiated into space, reaching a maximum and
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  Speaker 1: again decreasing with the current. But while the current increases,
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  Speaker 1: the electrostatic strain about the sphere decreases, thus sending out
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  Speaker 1: an electrostatic wave, which again increases as the current decreases.
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  Speaker 1: It will thus be seen that as these surges take place,
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  Speaker 1: alternate electromagnetic and electrostatic waves are radiated into space. Why
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  Speaker 1: do not these surges keep on forever? Mainly because the
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  Speaker 1: current in its flow across the gap encounters a resistance, and,
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  Speaker 1: instead of converting all its energy into magnetic waves, loses
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  Speaker 1: a portion as heat at each surge. This corresponds to
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  Speaker 1: oscillating the pendulum in a liquid or viscous material, the
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  Speaker 1: energy of its motion soon being converted into heat. So
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  Speaker 1: much for the creation of these waves, But how may
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  Speaker 1: their passage through space be detected? One way is by
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  Speaker 1: catching them on a wire. What is the manner of
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  Speaker 1: the catching? If a wire be placed so that it
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  Speaker 1: cuts the wave transversely to its line of motion, it
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  Speaker 1: is clear that the moving magnetic wave will induce in
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  Speaker 1: it an electromotive force. As these waves follow in rapid succession,
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  Speaker 1: a series of alternating electromotive forces is set up in
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  Speaker 1: the wire. These oscillatory currents are sometimes called jigs. How
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  Speaker 1: these jig currents make their presence known varies with the
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  Speaker 1: style of wave detector used Before entering upon the history
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  Speaker 1: of this spark telegray, as the Germans call it, it
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  Speaker 1: may be well to review some of the experiments which
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  Speaker 1: preceded this system. In eighteen thirty eight, Professor Joseph Henry
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  Speaker 1: of Princeton, making with an electrical machine and leydingjar a
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  Speaker 1: one inch spark in the top room of his residence,
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  Speaker 1: set up induced currents in the cellar of the same building.
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  Speaker 1: Professor Morse, however, was probably the first to successfully transmit
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  Speaker 1: signals without wire. On December sixteenth, eighteen forty two, he
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  Speaker 1: sent a wireless telegram across a canal eighty feet wide,
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  Speaker 1: and in November eighteen forty four, L. D. Gale, acting
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  Speaker 1: under instructions from Professor Morse, made wireless signals across the
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  Speaker 1: Susquehanna River at Havre de Grace, a distance of nearly
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  Speaker 1: one mile. In the latter experiment, mister Gale used as
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  Speaker 1: a source of energy six pairs of plates in the
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  Speaker 1: form of a galvanic battery. He found that the best
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  Speaker 1: results were obtained when on each side of the river
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  Speaker 1: two plates were immersed near its bank and worked connected
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  Speaker 1: by an insulated wire stretched along each shore for a
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  Speaker 1: distance three times as great as that which measured either
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  Speaker 1: path of the crossing signals. A few years later, James Lindsay,
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  Speaker 1: a Scotchman, repeated Morse's experiments, but without knowing of them.
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  Speaker 1: In eighteen fifty nine he read a paper before the
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  Speaker 1: British Association on the subject of telegraphing without wires, and
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  Speaker 1: among his hearers were Faraday and Lord Kelvin. A method
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  Speaker 1: of signaling without wires by means of the inductive effect
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  Speaker 1: of two parallel circuits was successfully used by Sir William
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  Speaker 1: Preece in eighteen eighty two. The principle of this method
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  Speaker 1: is as follows. If two loops of wire are placed
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  Speaker 1: parallel to each other, and an intermittent current is passed
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  Speaker 1: through one of them, waves of magnetic flux are sent
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  Speaker 1: out portions of which thread the second loop, and by
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  Speaker 1: their fluctuations produce currents in it, which may be detected
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  Speaker 1: by a telephone or other device. The strength of such
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  Speaker 1: signals falls off very rapidly from the source, and such
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  Speaker 1: a system can only be made to operate over short distances.
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  Speaker 1: Priests constructed two parallel lines, one on the English coast
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  Speaker 1: and one on the Isle of Wight, the arrangement of
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  Speaker 1: which is shown in Fig. Forty eight. It will be
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  Speaker 1: seen that the loop from which the waves emana is
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  Speaker 1: formed by the line, wire and the Earth. The crowning achievement, however,
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  Speaker 1: was the work of Hertz, whose early death deprived the
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  Speaker 1: world of the aid of a most powerful brain. In
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  Speaker 1: eighteen eighty six, Hertz discovered that if a loop of
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  Speaker 1: wire having a small air gap left in it were
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  Speaker 1: placed at a distance from a spark discharge of a
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  Speaker 1: Leyden jar minute sparks would pass across the air gap
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  Speaker 1: of the loop, thus indicating the presence of electric waves.
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  Speaker 1: He subsequently made a very complete study of the behavior
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  Speaker 1: of these waves, and thus gave a tremendous impetus to
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  Speaker 1: the development of wireless telegraphy. Second to the work of
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  Speaker 1: Hertz is that of Sir Oliver Lodge, who, in eighteen
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  Speaker 1: eighty nine to descis discovered the effect of electrical waves
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  Speaker 1: on the breaking down of the electrical resistance of two
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  Speaker 1: knobs barely in contact, which discovery resulted in the first
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  Speaker 1: means used to detect the presence of electrical waves, the coherer.
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  Speaker 1: The development of wave detectors is an important chapter for
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  Speaker 1: these instruments. Professor Fleming has suggested the use of the
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  Speaker 1: word simoscope as a general term including all classes of
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  Speaker 1: wave detectors. A great number of these have been invented,
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  Speaker 1: but they may all be included under the following classes.
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  Speaker 1: One spark seymoscopes, two contact cymoscopes, three thermal cymoscopes, four
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  Speaker 1: magnetic cymoscopes, five electrolytic cymoscopes, six electrodynamic cymoscopes, and seven
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  Speaker 1: vacuum tube cymoscopes. The first cymoscope invented was that used
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  Speaker 1: by Hertz in his investigation of electrical waves, and belongs
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  Speaker 1: to the first class. Resisted merely of a ring broken
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  Speaker 1: at one point and arranged so that the gap might
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  Speaker 1: be adjusted by means of a micrometer screw. Tiny sparks
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  Speaker 1: across this gap indicated the presence of waves. Since the
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  Speaker 1: electromotive force required to produce a spark across even a
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  Speaker 1: small gap is very considerable. It will be obvious that
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  Speaker 1: such a detector could only operate at a short distance,
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  Speaker 1: and would therefore be useless for the purpose of signaling.
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  Speaker 1: The next invention, made in eighteen ninety was the Bramley
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  Speaker 1: Coherer Fig. Forty nine, which consisted of a small glass
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  Speaker 1: tube containing two metallic plugs and separated by a gap
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  Speaker 1: partially filled with metallic fillings. This is an example of
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  Speaker 1: the second class. The metallic fillings, when loosely packed, offer
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  Speaker 1: a very high resistance to the passage of current through them,
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  Speaker 1: but the presence of waves breaks down their contact resistance,
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  Speaker 1: which continues after the waves have ceased. In order to
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  Speaker 1: again restore their resistance, they must be tapped or shaken,
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  Speaker 1: an operation known as decoherence. It is obvious that such
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  Speaker 1: an apparatus may be used like a key in a
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  Speaker 1: telegraph circuit, a key operated by electrical waves, and may
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  Speaker 1: therefore be used to operate a telegraphic instrument. Such was
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  Speaker 1: the first device used. It was defective, however, in that
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  Speaker 1: it was necessary to tap it after each signal. Decoherence
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  Speaker 1: was not certain. It required frequent adjustment, and the result
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  Speaker 1: was often a confused lot of signals. Many arrangements of
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  Speaker 1: loose contacts were tried, and the coherer was improved by Marconi, Lodge,
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  Speaker 1: Braun and others. One of the principal troubles being the
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  Speaker 1: operation of decoherence. Most of the inventors sought to develop
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  Speaker 1: a coherer which could be self restoring, and a number
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  Speaker 1: of successful types were invented. One of these was the
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  Speaker 1: Hughes coherer, employing carbon granules placed between iron plugs. The
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  Speaker 1: most perfect and successful of all these is, however, that
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  Speaker 1: devised by Sir Oliver Lodge and doctor Muirhead and used
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  Speaker 1: in the Lodgemere Head system, as shown in figure fifty.
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  Speaker 1: It consists of a steel disk slightly separated from a
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  Speaker 1: globule of mercury by a film of oil. The disc
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  Speaker 1: being arranged to rotate slowly. The presence of waves breaks
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  Speaker 1: down the oil film and establishes contact with the mercury,
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  Speaker 1: which contact immediately breaks upon the cessation of the waves.
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  Speaker 1: A siphon recorder placed in the series with the symoscope
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  Speaker 1: is used to record the message. Although a number of
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  Speaker 1: forms of magnetic style of detector were devised. It was
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  Speaker 1: not until nineteen o two that a successful working apparatus
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  Speaker 1: was produced. For some time before that, it was known
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  Speaker 1: that the oscillating currents received would annull wholly or in part,
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  Speaker 1: the magnetic hysteresis of iron when passed through a coil
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  Speaker 1: surrounding the iron. Hysteresis acts like molecular friction, so that
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  Speaker 1: when a magnetizing current is passed through a coil surrounding
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  Speaker 1: an iron core, the magnetization does not increase and decrease
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  Speaker 1: with the current in the coil, but lags behind it.
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  Speaker 1: If another coil be placed around this core and the
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  Speaker 1: oscillating currents passed through it, this hysteresis will be suddenly
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  Speaker 1: removed and the magnetism in the core will suddenly change
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  Speaker 1: in value. This sudden change could be detected by a
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  Speaker 1: third coil surrounding the core and connected to a telephone receiver,
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  Speaker 1: resulting in a sudden click. This was the principle of
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  Speaker 1: which Marconi made use in his magnetic detector, and which
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  Speaker 1: he has used in his long distance experiments. The arrangement
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  Speaker 1: is shown in Fig. Eighteen. It consists of a band
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  Speaker 1: of iron wires passing through two coils, one carrying the
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  Speaker 1: jig currents and one connected to the telephone. The wires
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  Speaker 1: are magnetized by two permanent magnets, and as they move
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  Speaker 1: under that portion where the two poles meet, the magnetic
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  Speaker 1: flux in them undergoes a reversal. Which reversal, however, always
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  Speaker 1: takes place at the same point until the jig currents pass,
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  Speaker 1: when the flux is suddenly shifted backward, causing a sound
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  Speaker 1: in the telephone. These detectors have come into very general
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  Speaker 1: use on account of their convenience, sensitiveness, and adaptation to
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  Speaker 1: rapid signaling. In nineteen o one, doctor Lee de Forest
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  Speaker 1: patented a detector which depends for its operation on the
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  Speaker 1: disruption of the minute metallic bridges or trees, which form
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  Speaker 1: under suitable conditions between the electrodes of an electrolytic cell.
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  Speaker 1: The apparatus, called a responder, consists in a glass tube
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  Speaker 1: similar to that of a coherer, in which are fitted
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  Speaker 1: two electrodes, preferably of tin. The space between the electrodes,
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  Speaker 1: about one sixty fourth of an inch, is filled with
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  Speaker 1: a viscous semiconducting liquid such as glycerine, with a small
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  Speaker 1: admixture of water, together with some peroxide of lead as
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  Speaker 1: a depolarizer to prevent the excessive evolution of gas. When
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  Speaker 1: a cell of suitable voltage is connected across this responder,
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  Speaker 1: metallic trees or bridges are formed which make a path
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  Speaker 1: of low resistance. But upon the passage of the jig currents,
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  Speaker 1: these trees are broken down and the circuit broken, producing
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  Speaker 1: a sound in a telefe phone receiver. Immediately upon the
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  Speaker 1: cessation of the jig currents, however, these trees again establish themselves.
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  Speaker 1: Another very successful and extremely sensitive detector was invented by
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  Speaker 1: Fessenden and Friedland and consists of a small platinum cathode
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  Speaker 1: containing nitric acid with a minute anode of platinum wire
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  Speaker 1: one ten thousandth of an inch in diameter. This little
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  Speaker 1: electrolytic cell, when polarized to the critical point by being
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  Speaker 1: connected to a battery, is remarkably sensitive to jig currents.
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  Speaker 1: It has been employed by Festenden in his transatlantic experiments.
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  Speaker 1: Many other detectors have been developed which have operated successfully,
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  Speaker 1: but those described are used most generally. It would not
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  Speaker 1: be possible, in a few words, to give a comprehensive
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  Speaker 1: idea of the various systems in use, as many important
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  Speaker 1: improvements which have been made in the last ten years.
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  Speaker 1: Those systems which have attained commercial importance are the Marconi,
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  Speaker 1: the Festen d, the de Forest, the slaviy arco and
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  Speaker 1: the Lodgemere head. The greatest differences are usually found in
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  Speaker 1: the receiving apparatus. The first system used by Marconi employed
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  Speaker 1: the coherer as a wave detector and the room wharf
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  Speaker 1: coil to produce the sparks from which the waves were radiated.
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  Speaker 1: The diagrammatic arrangement of the sending and receiving apparatus is
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  Speaker 1: shown in Fig. Fifty two, T being the coherer and
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  Speaker 1: l the telegraphic relay. Experiments with this form of apparatus
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  Speaker 1: were first made in eighteen ninety six in England, where
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  Speaker 1: Marconi went to obtain the assistance of Sir William Priest.
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  Speaker 1: These experiments were so successful that trials were made during
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  Speaker 1: the next year, at each of which something new was learned,
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  Speaker 1: whereby the distance of transmission was increased. From some of
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  Speaker 1: these experiments, Marconi worked out the effect of the height
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  Speaker 1: of the antenna or aerial wire on the distance of transmission.
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  Speaker 1: In August eighteen ninety seven, Marconi organized a company known
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  Speaker 1: as the Wireless Telegraph and Signal Company with a capital
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  Speaker 1: of five million dollars. In June eighteen ninety seven, Marconi
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  Speaker 1: went to Rome, and, after having undertaken in this city
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  Speaker 1: at the instigation of the Minister of Marine several experiments
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  Speaker 1: from one floor to another with a conductor three yards
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  Speaker 1: in height, was invited by the Honorable Brin, Minister of Marine,
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  Speaker 1: to undertake in the presence of a select commission composed
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  Speaker 1: of officers who were specialists belonging to the Royal Marines,
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  Speaker 1: some fresh experiments. The place chosen was the Gulf of Spitzia.
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  Speaker 1: The experiments took place between the eleventh and eighteenth of
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  Speaker 1: July eighteen ninety seven. The apparatus made use of for
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  Speaker 1: transmitting and receiving was similar to those employed on the
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  Speaker 1: Bristol Channel, that is to say, aerial wires ending above
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  Speaker 1: in metallic sheets. The coil was less powerful than that
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  Speaker 1: used in the former case, giving sparks ten inches in
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  Speaker 1: length only. The apparatus was located during the entire series
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  Speaker 1: of experiments in the electrical laboratory of Saint Bartholomew and
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  Speaker 1: bore an aerial line about seventy five feet in height,
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  Speaker 1: which was afterward prolonged to ninety terminating in a square
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  Speaker 1: metal sheet of about eight feet in the side. On
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  Speaker 1: the first three days, the eleventh, twelfth, and thirteenth of July,
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  Speaker 1: the experiments were executed on land, which gave very good
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  Speaker 1: results up to a distance of three and a half
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  Speaker 1: kilometers or say two miles. On the fourteenth of July,
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  Speaker 1: the receiver was set up on board a tug having
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  Speaker 1: a mast about fifty feet in height, which bore an
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  Speaker 1: aerial wire of equal length ending in a sheet about
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  Speaker 1: eight feet in the side. The transmitting station was bound
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  Speaker 1: to carry out the following instructions ten minutes after the
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  Speaker 1: start of the tug. It was to send for fifteen
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  Speaker 1: minutes dots and dashes at intervals of ten seconds, then
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  Speaker 1: transmit a phrase, maintaining between each signal and interval of
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  Speaker 1: ten seconds. Then to suspend transmission four in an interval
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  Speaker 1: of five minutes, after which it should go through the
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  Speaker 1: same round, but with intervals of five seconds instead of
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  Speaker 1: ten between each signal. The tug having started from the
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  Speaker 1: little port of Saint Bartholomew, the receiver registered some signs
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  Speaker 1: even before transmission had begun on land, a fact due
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  Speaker 1: doubtless to extraneous causes. She directed her course toward the
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  Speaker 1: western mouth of the Mole and continued to receive signals,
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  Speaker 1: not however, in the order and in the intervals that
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  Speaker 1: had been pre arranged, but much more frequently. The sky
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  Speaker 1: was covered with stormy clouds, and in the distance lightning
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  Speaker 1: was frequent. Hence it was surmised that beside the signals
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  Speaker 1: that were really transmitted, others, due to atmospheric influence, were
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  Speaker 1: impressing themselves, which rendered the strip of paper on which
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  Speaker 1: they were registered illegible. On again repeating these experiments, after
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  Speaker 1: the storm clouds had disappeared, correspondence came out very clearly,
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  Speaker 1: up to a distance of five thousand, five hundred meters
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  Speaker 1: nearly three miles with a tug stationary. The tug was
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  Speaker 1: again put in motion so as to interpose between itself
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  Speaker 1: and the station of Saint Bartholomew, the point called La Castagne,
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  Speaker 1: in order to ascertain what effect such a screen would
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  Speaker 1: have on signaling. The signals ceased as soon as the
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  Speaker 1: obstacle intervened to recommence on the tug being moved from
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  Speaker 1: its influence. On the return journey, the messages continued to
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  Speaker 1: come out clear and exact. On the seventeenth of July,
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  Speaker 1: trials were made from the same stations of Saint Bartholomew
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  Speaker 1: to the armored ship San Martino, anchored at a distance
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  Speaker 1: of about one on three quarter miles from the transmitting station,
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  Speaker 1: the aerial conductor of which had been carried to a
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  Speaker 1: height of about forty yards, while the ship bore at
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  Speaker 1: the receiver an aerial line first of twenty and then
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  Speaker 1: of thirty yards in height. Transmission succeeded perfectly independent of
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  Speaker 1: the position of the coherer and the receiver, that is
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  Speaker 1: to say, even if they were screened at the sending
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  Speaker 1: station and surrounded by metallic masses under cover or placed
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  Speaker 1: below the waterline in the ship. End of Section thirty one.