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tial principle so successfully applied by Häuy to the discovery of magnetism in minerals containing traces of iron, can be had recourse to here so as to enhance still further the delicacy of these beautiful instruments.
Having thus sufficiently exposed the construction and mode of action of the galvanometer, I must reserve for another place, the results of my experiments in testing the value of the different methods described.
§ 4.-Henry's Magnetic Telegraph.
I have still however to notice another proposal which has attracted great attention, and is said, on good authority, to be in course of practical application in the United States.
Professor Henry proposes to employ the sudden development of magnetism, occasioned in a horse shoe bar of soft iron while surrounded by a spiral of insulated wire, the extremities of which are in contact with a voltaic couple. The magnet thus created attracts a light piece of iron which carries an arm. The arm when attracted marks dots on a revolving cylinder, or may strike a bell. The arrangement is shewn in the following figure. The spiral wire in the centre is a spring to lift up the arm on the cessation of each stroke.
Eleven miles of wire were employed in one of Henry's experiments, but the wire was coiled spirally round a drum, a circumstance which considerably invalidates the results. This will seem sufficiently intelligible by reference to the construction of the "coil electro-magnetic machine," described in a subsequent page.
§ 5.-Experiments by the Author.
I have now given an adequate sketch of the several methods of communication hitherto proposed, and I proceed to the description of the experiments I have carried on, in the view of testing the comparative merits of the preceding plans and of another, which I have myself devised.
My first object was to construct a line of wires of sufficient length to afford practically valuable results. With Dr. Wallich's liberal aid a parallelogram of ground, 450 feet long by 240 in breadth, was planted with forty-two lines of bamboos. Each line was formed of three bamboos firmly driven into the ground, fifteen feet in height. Each row was disposed so as to receive half a mile of wire in one continuous line, thus,*
The strands of wire were one foot apart from each other. row was laid down, it was carefully coated with tar varnish. A tent was pitched in front of the entire line, and the connections of the wires were so established that in the course of half an hour it could be tested from centre to the extreme flank, so as to ascertain the effects of lengths of wire, varying from one to eleven miles at either side, forming a total circuit of twenty-two miles. This may be perhaps more readily intelligible from the subjoined figure.
The cross lines above the numbers exhibit the wires led from each half mile of conductor. Thus by cutting through 1. 1. the next numbers to right and left became the conductors or nearest electrodes, and the length of the circuit thus rose from one to three miles; cutting 2. 2. will make 3. 3. the electrodes, and increase the circuit to five miles, and so on, each section added a mile to the circuit at either side.
The wires employed were of iron (annealed), diameter one-twelfth of an inch. It is almost needless to observe that iron was used not from choice but necessity. A sufficient quantity of copper wire was not procurable in Calcutta, no draw-bench was ready to manufacture the necessary supply, moreover the rainy season was fast approaching when such experiments could scarcely be attempted, constant exposure in the open air being essentially requisite to success. The expense
again of copper would have amounted to much more than a private individual could afford.
With iron wire however I considered that the results would be still of much practical value. Being the worst of the metallic conductors of electricity, it seemed a reasonable inference that whatever might be found practicable with iron, would à fortiori be so with the copper, or best conductor.
On the completion of the line the following instruments were tried. 1st. An electro-magnet of soft iron, 1 inch in diameter, poles 1 inch apart, length from centre to poles 12 inches, weight 14 lbs. surrounded by one hundred yards of insulated copper wire, the twelfth of an inch in diameter. This electro-magnet, when excited by the voltaic battery used in the subsequent experiments, with conductors seven feet in length, supported 240lbs.
2nd. An electro-magnet of very small size, constructed by Watkins, of London, capable of supporting 30lbs. with the battery now referred to, and with the same length of conductors.
3rd. An astatic galvanometer by Watkins and Hill, already referred to.
4th. An electro-magnetic induction machine, also by Watkins, of which a brief description is desirable.
This instrument consists of a coil of thick wires insulated by silk, and wound spirally round a wooden cylinder having a hollow axis one inch in diameter. The ends of this coil are connected with metallic screws, so that they can be joined to the poles of a voltaic battery.
Around this primary coil is wound a second coil of extremely thin wire, also insulated and 1000 yards long, totally unconnected, though in close juxtaposition with the primary coil, the ends of the wire being led to screws to which handles, directors, &c., can be attached, thus,
Into the hollow axis at a is introduced a bundle of insulated iron wires.
The action of the instrument may be very briefly described. While the battery at b is in contact with the wires c d the primary coil is excited. By interrupting the circuit at + or elsewhere, at the instant of its interruption, the secondary or external coil becomes excited by induction or proximity-and this excitement is augmented by the influence of the magnetism simultaneously annihilated in the central bundle of iron wire.
The electrical state thus momentarily generated in the secondary wires, may be rendered evident by the production of a spark and shock, by effecting chemical decomposition and all the other signs of electrical action, at the terminations of the secondary coil e, ƒ.
In this cursory description I confine myself to facts alone, and refrain from entering on any theoretical speculation as to the causes of these singular and deeply interesting phenomena.
Experiments with the Electro-magnet No. 1.
The day being fine, the ground and bamboos perfectly dry, at 9. A. M. the sustaining power of the electro-magnet No. 1. was tested with iron conducting wires ten feet long, and found to amount to 46 lbs. With one mile of same wire, mile at each side,
4 Miles, no sustaining power. Assuming iron to be inferior to copper in about the proportion of 1 to 7, according to Sir Humphry Davy and Becquerel's standard of conductors, this experiment shews that for equal diameters of wire, copper would convey the signal by Henry's method to about twenty-one miles in an imperceptible period of time. This distance might be extended by enlarging the diameter of the wires, but to what limit, is as yet unknown.
Experiments with Galvanometer.
The astatic galvanometer was arranged and levelled with much care, the needles retaining a very slight degree of directive force so as to cause them to swing in the magnetic meridian.
At 1 Mile, deviation maximum or ..