This useful instrument, the invention of which is based upon Oersted’s
discovery of the deflection of the magnetic needle, by the action
of conducting wires conveying galvanic currents, seems to have
furnished to most of the inventors of telegraphs, the main spring of
communication. It was a very natural suggestion, as being the most
convenient and ready mode of obtaining the required motion, by making
and breaking the galvanic circuit. Thus Steinheil, Wheatstone and
Bain have availed themselves of this _one idea_ to effect that part
of the telegraphic operation which may be called the _galvanic_, in
contradistinction to the _mechanical_ parts, which last have varied
considerably with different operators. The construction and operation
of the galvanometer may be understood by reference to the figures 27,
28, 29. A A, fig. 28, are two long coils of covered copper wire, a side
view of which is shown in fig. 27. These coils are connected with the
binding screws, L L, attached to the frame, or box, holding the coils.
Two coils are used for the convenience of allowing the pivot sustaining
the magnetic needle to pass between them; one coil might be used, by
leaving room enough between the wires for a socket for the pivot,
but the arrangement, represented, is the most readily constructed. A
side view of the instrument, figure 27, shows the arrangement of the
needles, two of them being generally used to increase the operation of
deflection, and to neutralize the influence of the earth’s magnetism.
The pair of needles is usually denominated, an _astatic_ needle, or a
needle without directive power; as the current traversing a conducting
wire gives different directions to needles placed above and below the
wire, the action upon the two needles thus placed is combined, by
arranging their poles in opposite directions. When the current is in
the direction indicated by the arrows in figure 27, the north pole
of the needle, within the coil, is carried in a direction from you,
as you face the drawing, and the north pole, without the coil, in a
contrary direction. The operation upon the south pole is the reverse.
Changing the direction of the galvanic current, reverses the motions.
It is usual to apply the force of torsion, or of a _hair_ spring, or
of the superior weight of one extremity of the needle, to act against
the deflective force of the current, and to attach a graduated scale to
the instrument, fixing it between the uppermost needle and the coils as
in figure 29. Instead of deflecting the needle, the coils themselves
may be deflected, as in the galvanoscope of Prof. Page, invented in
January, 1837, and described by him in the 33d vol. of Silliman’s
Journal, page 376. The object of this contrivance was to enable him
to use powerful magnets and lighter coils. This modification of the
galvanoscope, Mr. Bain has preferred as the means of operating his
telegraph.
[Illustration: FIG. 27.]
[Illustration: FIG. 28.]
[Illustration: FIG. 29.]
_An Interesting Experiment of Supporting a Large Bar of Iron within the
Helix. Discovered by Mr. Vail, January, 1844._
It has been shown, many years since, that a magnetic needle would be
drawn into and suspended within a helix, conveying a galvanic current,
and that in the case of using large bar magnets, the coils or helices
might be made to move over them, as in De La Rives’s rings; but in no
instance, I believe, has it been recorded, or observed, that a bar of
iron weighing a pound or more, could be drawn up into the helix and
there sustained in the air, as it were, without support. If the helix,
as shown in figure 30, be connected with from 6 to 12 pairs of Grove’s
battery, the bar may be drawn up into its centre and there sustained in
a vertical position by the action of the helix, forming an exceedingly
interesting and paradoxical experiment.
[Illustration: FIG 30.]
[From the National Intelligencer.]
APPLICATION OF THE ELECTRO MAGNETIC TELEGRAPH TO THE DETERMINATION OF
LONGITUDE.
Among the wonderful developements of the new telegraph, one has just
came to light which will be regarded in the world of science as deeply
interesting. Prof. Morse suggested to the distinguished Arago, in 1839,
that the electro magnetic telegraph would be the means of determining
the difference of longitude between places with an accuracy hitherto
unattainable. By the following letter from Capt. Charles Wilkes to
Prof. Morse, it will be believed that the first experiment of the kind
of which we have any knowledge, has resulted in the fulfilment of the
Professor’s prediction.
WASHINGTON, _June 13, 1844_.
MY DEAR SIR—The interesting experiments for
obtaining the difference of longitude through your magnetic
telegraph were finished yesterday and have proved very
satisfactory. They resulted in placing the Battle Monument
square, Baltimore, 1 _m_, .34 sec. .868 east, of the
capitol. The time of the two places was carefully obtained
by transit observations. The comparisons were made through
chronometers and without any difficulty. They were had in
three days, and their accuracy proved in the intervals
marked and recorded at both places. I have adopted the
results of the last day’s observations and comparisons,
from the elapsed time having been less.
The difference of the former results, found in the
American Almanac, is .732 of a second. After these
experiments, I am well satisfied that your telegraph offers
the means for determining meridian distances more accurately
than was before within the power of instruments and observers.
Accept my thanks, and those of Lieutenant Eld, for
yourself and Mr. Vail, for your kindness and attention in
affording us the facilities to obtain these results. With
great respect and esteem, your friend,
CHARLES WILKES.
Professor S. F. B. MORSE,
_Capitol, Washington_.