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Up to the sixth mile the needles were deflected with great rapidity on the connexion being made with the voltaic element. The reversal of the order of connection also satisfactorily reversed the needle from east to west, and the contrary. But when the deflection fell to below 40°, the movements were exceedingly sluggish, so that on an average two seconds elapsed before each signal could be read off. The change of battery poles then often failed in reversing the direction of the needles and here, as before, at least two seconds were consumed in each movement. Applying the same rule to this as to the preceding experiment, the galvanometer would convey signals by a copper wire to a distance of twenty-eight miles-and this might be increased by enlarging the wire or the battery, or by adding to the delicacy of the galvanometer -but in one essential point this system was deficient, namely, in rapidity of movement. Two seconds or even one, on each telegraphic movement, would be an extravagant waste of time compared with the celerity with which signals can be conveyed by another method.
One of the most delicate of all tests of voltaic electrical action is the decomposition of ioduret of potassium and the production of a blue colour which the free iodine strikes with starch. This effect was produced in my experiments for a line of three miles of wire. Beyond this no decomposition could be effected. From this fact we are entitled to infer the impracticability of Soëmmering's method. See § 2.
Induction machine, and mode of correspondence by Pulsations and Chronometers.
The battery was connected with the primary coil see fig. 1. p. 723. by very short wires ;-the ends of the secondary coil wires screwed to the right and left wires of the great parallelogram. P. 721.
On breaking contact with the primary coil, a shock utterly intolerable passed at half a mile to an individual holding the metallic handles in which the wires ended.
To avoid fatiguing details I may at once state, that by this secondary coil, excited by but three small voltaic couples, the shocks up to seven miles were exceedingly smart-at eleven and a half at each side, they amounted to no more than strong, but not disagreeable, sensations. I think these might be best termed " pulsations," for to the hand they impart the same feeling proportionately, that a strong and full pulse does to the finger.
Each pulsation is practically simultaneous with the breaking of contact with the battery. To give a rude idea of the velocity of the signal, the contact being broken by a clicking wheel, on a perfectly calm morning, at a distance of but sixty yards, the pulsation was invariably felt at a sensible interval before the click which preceded it was heard. Thus sound travelling at the rate of 1090 feet in one second to 121 feet in one-ninth of a second, the electrical impulse passes through a total circuit of twenty-two miles, in less than that practically insignificant fraction of time. This however conveys but an erroneous notion of the almost inconceivable velocity of the impulse. Professor Wheatstone has proved that the electrical accumulation of the Leyden phial is discharged and circulates through copper conductors, one fifteenth of an inch in diameter, with greater velocity than the progress of light through the planetary spaces, and in the rate at least of 288,000 miles in a second. Now the discharges of the Leyden bottle and those of induced coil electricity are in the closest circumstances analogous to each other.
Of the pulsations thus transmitted, it is perfectly easy to count six in one second-thus with a little practice any signal number can be made from one to six in one second.
Besides the simple repetition of the pulsations up to nine, beyond which they become indistinct for each signal, there are at least two modes of conveying other sensations by this apparatus. If the connexion between the battery of the primary coil be made and broken by a ratchet-wheel of brass and silver, and the wheel be turned pretty rapidly, a sensation analogous to the ruffle of a drum is so distinct as to render mistake impossible. A third set is obtained by interposing a flat file in the battery circuit, and interrupting this by drawing one wire along the surface of the file; here instead of the ruffle, the feeling is that of a blunt saw drawn lightly across the palms of the observer's hands. It is difficult to express in words the differences in these distinguishing signals, but the practice of a quarter of an hour will make the observer so familiar with them, that he can without the slightest difficulty carry on a communication by numbering or spelling with his distant correspondent. With a tithe of the practice of a pianist or harpist, the most perfect sympathy is practicable between the signalists, and that as fast as the signal can be spelt. In short, with but little less velocity than the articulations of language or the writing of stenographic characters, this silent, but thoroughly intelligible, and still most secret of all correspondence can take place.
It is almost unnecessary for me to remind the reader of the admitted
fact, that the exquisite delicacy of the impressions of the touch transcends, in some respects, the evidence of all the other senses. The eye and ear are liable to distraction by casual sounds or phenomena, while the attentive touch knows no interruption. The eye must close momentarily and thus lose the observance of many rapid phenomena. Dazzled by too vivid lights, and confused by too constant watching, vision soon ceases to be accurate, while the frequent repetition of similar sounds either becomes absolute silence to the ear, or like the murmuring of a rivulet or the humming of insects, gradually narcotizes the observer. Let the experimentalist attempt to count but 200 rapid strokes of a faint bell, and he will at once acknowledge the imperfections of any acoustic method.
Thus with copper conductors equal in diameter to the iron wires I employed, signals by pulsation are proved to be communicable by the method above described, in less than any appreciable period of time, to the distance of 154 miles.
Besides the method of telegraphing by pulsations and other signals recognized by touch alone, there is another of which I have made extensive trial, and which is capable of affording still more accurate and intelligible and equally rapid results.
Without any knowledge of the experiments quoted by Steinheilsmany months indeed before the paper by that author was published in England-I attempted, and with success, to effect the transmission of signals by using time-keepers at each terminus, and causing the pulsation to be felt as the hands simultaneously passed a certain number or letter on the dial.
In these experiments I first employed a pair of watches modified for my use by that ingenious artist Mr. Grant, of this city. All the movements were taken out but those connected with the second-hand, and a long lever was so constructed as to check the balance-wheel at pleasure during the recoil. Round the second-hand was placed a card dial laid off with three concentric circles divided each into twenty parts. Omitting vowels and superfluous letters, the alphabet was laid down in each circle so that the hand would during each revolution point to any letter three times; the compartments were moreover numbered on the same principle, so that each figure from one to ten would be pointed to six times in a revolution.
The hand is passing each compartment during three seconds. The observer receives say two pulsations, and is thereby referred to the second circle, and reads the letter or cypher, according as the signal be for spelling or numbering.
Although the watches were of the very cheapest kind, and would not keep time together for more than five minutes, still they were quite sufficient to enable a correspondence to be carried on. Thus a signal seldom lasted longer than three minutes; both watches were then allowed to run to No. 1 or zero, and stopped. To renew correspondence a prolonged roll was communicated. If but one roll, it indicated spelling; if two, numbering. On the roll ceasing, three pulsations at intervals of one second were passed, and at the third the correspondents started their watches.
The pendulum was also tried, and with decided advantage. Two German clocks sufficed to demonstrate the practicability of the system. The striking parts were removed, and also the hour and minute hands and dial.—To the axis of the escapement wheel a needle was attached, carrying a light hand which indicated on a dial the signals above described. The German clocks (which cost but 16 rupees the pair,) in numerous experiments beat together for several hours, and could always be relied on for one hour at least. It is almost needless to add, that by shortening or lengthening the pendulum the rate was readily varied from 40 to 80 seconds for each revolution.
I did not omit chronometers, although I could not of course so alter these costly instruments as to adjust them perfectly to my experiments. It is obvious however that chronometers will on my method give an unerring and constant mode of telegraphic correspondence. In a recent trial at Greenwich the mean error of several instruments in one year was but two seconds!* Here then are two movers constantly and simultaneously pointing to one and the same signal, be it letter, figure, or cypher. The electric pulsations which "take no note of time" or distance, supply us with the means of converting this synchronism to the unexpected and invaluable end to which it is now proposed to be applied.
Even employing inferior chronometers, the addition of a moveable dial which could be adjusted daily on a method too simple to need description, would secure the perfection of the correspondence; or the daily difference of the instruments being known, a tabular correction. could we made; or, lastly, by an occasional astronomical observation of true time at each station, the object in view could be as certainly obtained.
In 1831, the first three prize chronometers only differed