Tele-graph′ic a-larm′.
（
Telegraphy.) A sounder operated by electro-telegraphic means.
There are many forms:—
The alarm which runs down, a detent being withdrawn from the escapement.
See acoustic Tele-Graph, page 11.
The clicking-instrument, operated by successive impulses.
This has grown into the ordinary telegraph, which is read by sound.
See also telephone.
Although
Franklin, in 1748, fired spirits by means of a spark transmitted across the
Schuylkill, the first distinct plan for a telegraphic alarm to call the attention of an operator or correspondent was by Schweigger, about 1811.
He proposed a pistol, charged with a mixture of oxygen and hydrogen gases, to be fired by a spark derived from the long electric wire proceeding from the distant station.
 |
|
Telegraphic alarm. |
Fig. 6239 is an alarm with an escapement.
a is an electromag-net;
b an armature of soft iron, which is attracted as often and as long as the voltaic current circulates through the coil, but is prevented from coming in actual contact therewith by means of two copper studs tipped with ivory.
The armature is mounted on the short arm of a lever
c, which is normally pressed by a spring so as to engage a stop on the wheel
d, preventing it from moving.
When a current passes through the coil, the armature, being attracted, releases the lever from the stop, and clock-work mechanism operating through the scape-wheel
d upon the pallets
e causes the hammer
f to vibrate, striking each side of the bell
g alternately.
 |
|
Magneto-electric telegraphic alarm. |
In
Fig. 6240, the hammer is rung without the intervention of an escapement, by means of a current of magneto-electricity.
The cores of two coils
a a are connected by a bar
b, and serve as keeper to the horseshoe magnet
c. On depressing the lever
d, these are raised from the poles of the magnet, creating a current which passes through the coils and along the line to the bell.
 |
|
Connection for bell-alarm. |
 |
|
Clock-work alarm. |
Fig. 6241 exhibits the connections for a bell-alarm.
a is the battery;
b, the zinc-pole;
c, the carbon-pole;
d, the key;
e f,
[
2507]
the binding screws of the bell-magnet.
When the key is depressed, the battery current is thrown upon the bell.
The three elements of the circuit may of course be separated by any distance within that through which the battery is able to transmit its current.
A sounder or register may be substituted for the bell.
In the alarm, of which a side view is shown at
B, and front and back views at
A C,
Fig. 6242, two electro-magnets
c c, having their coils connected and wound in similar directions, are joined at one end by a piece
d of soft iron.
At the other end is pivoted a soft iron armature
a, placed at such a distance as to be strongly attracted by the electro-magnet when the circuit is completed through its coils, and is connected with a detent
e, which ordinarily engages a fly, but is released when the armature is attracted by the closing of the circuit, allowing a train of clock-work, impelled by a spring or weight to operate, and through the medium of a scape-wheel and pallets, causing the hammer
h to strike the bell
g. When the current ceases to flow, the armature is retracted by the spring
s, again locking the mechanism.
