In′di-cator-tel′e-graph.
An electric telegraph in which the signals are given by the deflections of a magnetic needle.
It was about 1819 that Oersted of
Denmark made the discovery that if a magnetic needle, free to turn about its center, were placed near to and parallel with a wire, then, on causing an electric current to pass through this wire, the needle would be deflected to an angle proportioned to the force of the current.
In
Fig. 2670,
A, a b being the wire and
n s the needle, the passage of a current in the direction of the arrow deflects the needle to the position indicated, the stops
c c limiting its motions.
When the direction of the current is changed, the
n (north) pole of the needle is deflected in the other direction.
By a preconcerted system, a given number of movements to right, left, or a certain number in alternate directions, a system of indications representing letters and numerals is obtained, as will be explained presently.
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Indicator-telegraphs. |
In practice, a pair of needles, rendered astatic (see astatic needle), are used and placed in conjunction with a coil of wire, which multiplies the transmitted current so as to cause a much greater deflection than that due to the line current itself, as shown at
B, which consists of a frame around which are wound several hundred feet of fine copper wire.
The needles are mounted on the same axis, one within and one without the coil, and the outer one is exterior to the dial-plate of the telegraph.
Following the discovery of Oersted,
Ampere in 1820 suggested that the needle moved by the galvanic current should be used for conveying signals, and the idea was elaborated in lectures by
Ritchie in 1830.
Baron Schilling, in
Russia, in 1832, contrived a complicated instrument on this principle, in which a separate circuit and needle was employed for each
[
1182]
letter and numeral, 36 needles being employed, each being excited by a current of electricity as the occasion required.
Cooke and
Wheatstone's indicator-telegraph, patented June 12, 1837, is the most prominent and best example of this class of instruments, and is generally used in
England.
America and the continent of
Europe employ the much superior system of
Professor Morse, introduced about the same time.
C is a front and
D a rear view of
Wheatstone's instrument.
In this the right and left hand terminals
a b of the multiplier are each connected to the linewire, which is, when a message is not being transmitted, thrown out of connection with the local battery.
The circuit is opened and closed by means of a drum
c, at each end of which is a stout steel wire forming respectively the positive and negative poles of the battery; these wires, on turning the drum, are brought in contact with springs connected with the line-wire, by which either a positive or negative current is transmitted, causing the needle at the receiving station to be deflected to the right or left accordingly.
The letters of the alphabet and other signals are indicated by the number of beats made by the needle, some to the right and others to the left; thus “D” is represented by a beat to the right and one to the left.
“L” requires four beats, in the following order, right, left, right, left, and so on for other letters.
The double-needle telegraph on the same principle is much more expeditious, and has been extensively employed in
England, but requires two linewires.
See electro-magnetic telegraph.
