English experiments
Again in 1885, Preece arranged numerous experiments with a view to testing the properties of induction in telephone wires to determine to what distance parallel wires could be separated before the inductive influence ceased to operate. Two separate squares (the sides of which were 440 yards in length) of insulated wire were laid on the Town Moor at Newcastle, parallel to each other and a quarter of a mile apart (Fig. 3). At this range, communication was easily established between the two circuits, and even when the squares were separated by I,000 yards the inductive effects were still appréciable. It was found, however, that when the distance between the parallel wires excceded the length of the wires themselves, the strength of the inducted current was considerably diminished.
Several similar trials were held (in I886) in different parts of England. Between Durham and Darlington, the ordinary working currents in one line were clearly heard in a telephone on another line, running parallel but some miles distant. Similar inductive effects were obtained on the east and west coasts between Newcastle and Gretna on lines even 40 miles apart.
In 1892 a royal commission was appointed to inquire into the practicability of electric communication between the shore and lighthouse and lightships. They authorized Preece to proceed with his proposed scheme, in order to test the theories he had formed as a result of his numerous experiments. The Bristol Channel was selected as being a suitable place for the experiment, for here are two islands, Flatholm and
(page 427)
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Steepholm, distant from Lavernock Point three and five miles respectively (Fig. 4). Communication was easily established over the shorter distance (3.3 miles), but between Lavernock and Steepholm (5.35 miles) conversation was found to be impossible; and Morse signals, although perceptible, were unreadable. In March 1898, Preece's system was permanently established between Lavernock Point and Flatholm, and was handed over to the British War Office. A few months later S. Evershed's relays were added to work a call-bell, making the system "complete and practical."
Although Preece's system gave great promise, its limitations were soon realized. It was found that, as the distance between the two wires increased, the length of the wires had to be increased also; and that it was necessary for the length of each wire roughly to be equal to the distance between the two. Thus, although quite successful for communication over short distances, this system was useless for long distances on account of the great lengths of wire necessary for its successful working.
(page 428)
Editor's note: The graphic below is not part of the original article but it describes an important point
An experiment in electromagnetic induction: Two coils of wire are wrapped around a nail. The coils are insulated from the nail itself by several pieces of paper, which you cannot see in the drawing. When the battery is connected current steadily flows in one direction and no sound is produced. Remove a lead from the battery and a clicking noise sounds from the receiver. Current in one wire has been induced to flow in the second wire. Only when the current is turned on or off do you get a change in the electromagnetic field and a click.