Lives of the electricians

by William T. Jeans

Nor did his fertility in illustrating visual effects end here. Mr. J. Plateau stated in the journal of the Belgian Royal Academy for 1851 that Professor Wheatstone had communicated to him a plan for combining the principle of the stereoscope with that of the Phenakisticope, whereby figures simply painted upon paper would be seen both in relief and in motion, thus presenting all the appearances of life.

In 1851 he supplied the scientific world with a mechanical illustration of the earth’s rotatory motion which was much admired, and which set at rest some disputed points. Questions had been raised at that time as to the effect which the rotation of the earth had upon bodies which, like the pendulum, oscillated from fixed points; and M. Foucault designed mechanical means of showing such effects which were said to make the rotation of the earth as evident to the sight as that of a spinning-top. His original experiment was shown in Paris to M. Arago and other scientific men, and was described as follows:—To the centre of the dome of the Pantheon (272 feet high) a fine wire was attached, from which a sphere of metal, four or five inches in diameter, was suspended so as to hang near the floor of the building. This apparatus was put in vibration after the manner of a pendulum. Under, and concentrical with it, was placed a circular table, some twenty feet in diameter, the circumference of which was divided into degrees, minutes, &c., and the divisions were numbered. The elementary principles of mechanics showed that, supposing the earth to have the diurnal motion upon its axis which explains the phenomena of day and night, the plane in which the pendulum vibrated would not be affected by this diurnal motion, but would maintain strictly the same direction during twenty-four hours. In this interval, however, the table over which the pendulum was suspended would continually change its position in virtue of the diurnal motion, so as to make a complete revolution in about 30h. 40m. Since, then, the table thus revolved, and the pendulum which vibrated over it did not revolve, a line traced upon the table by a point or pencil projecting from the bottom of the ball would change its direction relatively to the table from minute to minute, and from hour to hour; so that when paper was spread upon the table, the pencil formed a system of lines radiating from the centre of the table; and the two lines thus drawn after the interval of one hour always formed an angle with each other of about eleven and a half degrees, being the twenty-fourth part of the circumference. This was actually shown to crowds who daily flocked to the Pantheon to witness this remarkable experiment. The practised eye of a correct observer, aided by a magnifying glass, could actually see the motion which the table had in common with the earth under the pendulum between two successive vibrations, it being apparent that the ball did not return precisely to the same point of the circumference of the table after two successive vibrations.

This experiment was repeated in other towns both on the Continent and in England with accordant results. It was pointed out, however, that the influence of the earth’s magnetism and other sources of error might produce discrepancies; but Professor Wheatstone invented an apparatus which presented a complete illustration not only of the general principle, but of the precise law of the sine of the latitude. He maintained the principle that so long as the axis of vibration continues parallel to itself, the arc of vibration will continue parallel to itself; but if the axis does not continue parallel, the direction of the arc of vibration will deviate. His apparatus illustrated that principle. Instead of a pendulum he used the vibrations of a coiling spring, the axis of which could be placed in any required inclination or latitude with respect to a vertical semicircular frame which revolved about its vertical axis: the direction of the vibration was seen to change in a degree proportioned to the sine of the latitude or inclination. He remarked, with reference to Foucault’s experiment, that the difficulty of the mechanical investigation of the subject, and the delicacy of an experiment liable to so many causes of error, had led many persons to doubt either the reality of the phenomena or the satisfactoriness of the explanation; and he therefore supplied an experimental proof which was not dependent upon the rotation of the earth. His experimental proof was pronounced the most complete and satisfactory that had been given.

Another subject that attracted his attention for years was the art of writing in cipher. When he was before a Parliamentary Committee in 1840 he was asked whether the telegraph was not open to the objection that the officials working it necessarily became acquainted with the contents of all the messages. His only reply to that objection then was that secret messages could be sent in cipher. In later years he constructed a machine for that purpose, intending to complete the benefits of the electric telegraph by rendering it possible to transmit telegraphic messages in a way that would render their contents unintelligible to the officials through whose hands they passed. This machine was called the cryptograph, and it periodically changed the characters representing the successive letters of the written communication, so that it could not be read except by the receiver, who, possessing a corresponding machine set in the same way as the sender’s, could by reversing the operation understand the characters. He stated that by the aid of this instrument an extensive secret correspondence could be carried on with several persons, and a separate cipher could be employed by each correspondent. The cipher despatches prepared by it were unintelligible to any person unacquainted with the word that might be selected as the basis of the cipher alphabet, and though any person might possess one of the instruments, he could not translate the cipher so long as the key-word was kept secret. Although this instrument has been scarcely known to the public, experience has proved its simplicity and efficiency; and it has been employed by the British Government, the French Government, and the English police.

Its principle is easily understood. Any word in which the same letter does not recur, may be selected as the key-word. Take the word “saucer,” and write under the separate letters of it, the remaining letters of the alphabet consecutively in the following columnar form:

In the machine are two movable spaces, one containing the letters of the alphabet in the usual order, and the other adapted to receive in juxtaposition the cipher letters which, with “saucer” as the key-word, would be the above letters arranged in a row, one column following another, thus:

A marvellous instance of his skill in deciphering cryptographic documents occurred in 1858. Sir Henry Ellis relates that a good many years previously the trustees of the British Museum purchased at a high price what appeared to be a very important document in cipher, occupying seven folio pages closely filled with numerals. The top of every page bore the signature of King Charles the First, and was countersigned by Digbye. For a long time Sir Henry Ellis endeavoured to get it deciphered for the purpose of including it in his series of letters illustrative of the history of England, but he could not get any one able to read it. One evening at Earl Stanhope’s he accidentally mentioned that fact to Lord Wrottesley, who suggested that Professor Wheatstone’s ingenuity might be able to unravel the secret writing, and accordingly Sir Henry Ellis at once sent it to the Professor, requesting that he would investigate its contents. This took place on June 1st, 1858. In the document in question about ninety different numerals were employed to represent the letters of the alphabet, and besides the complexity of each letter being represented by several distinct numerals, there was no division between the different words, and the numbers represented not English (as was at first supposed) but French words. This document, which had baffled all other experts, was interpreted by Professor Wheatstone. A copy of it having been sent two or three years afterwards to the Philobiblon Society, along with the key to the cipher, the Society expressed “their admiration of this additional instance of that wonderful faculty of interpretation which seems to ordinary minds a special intuition not unworthy of a great scientific discoverer and practical benefactor of the age.”

Among the subjects that engaged his attention both at the beginning and the close of his electrical studies was the construction of self-registering thermometers. In 1843 he invented a telegraphic thermometer, or rather an electro-magneto-meteorological register. It recorded the indications of the barometer, and the thermometer, and the psychrometer every half-hour, and printed the result in figures on a sheet of paper. The recording mechanism was a kind of clockwork, which was capable of registering 1000 observations in a week without any readjustment, and it could be prepared in five minutes for another week’s work. In consequence of this periodic winding up, the instrument could not be left for an indefinite time; and as there were many situations in which it was desirable to have meteorological indications, but to which access could not be obtained for long periods, he devised a new telegraphic thermometer whose indications were made visible at distant stations without the aid of clockwork. It consisted of two parts; one part, called the responder, contained a metallic thermometer consisting of a spiral ribbon of two dissimilar metals; this responder was connected by two telegraph wires with the other portion of the apparatus called the questioner, which recorded the changes of temperature by the movement of a hand on a dial round the edge of which was a thermometric scale. The responder could be placed at the top of a high mountain for any length of time, while its indications could be read at the station below; it could be placed deep down in the earth whose temperature could thus be ascertained over a long period; or it might be lowered to the bottom of the sea, and its indications read at intervals during its descent as well as periodically at the bottom, whereas previous marine thermometers required to be raised at every fresh observation.