The telegraph manual: a complete history and description of the semaphoric, electric and magnetic telegraphs of Europe, Asia, Africa, and America, ancient and modern.

USE OF BELL IN TELEGRAPH OFFICES. 353
When the current is coming over the line, and traversing the
electro-magnet A, the armature being attracted, makes the current of the local battery pass into the bell apparatus, and at the
same instant disengages the rod a b, which rises under the action of the spring d.
Fig. 6.
All the relays may be arranged in a single box, in order to
save room. A single local battery being necessary, all the
screws, such as Mr, communicate together, as well as all the
armatures. Above each of them is written the name of the station with which it is in connection. For these bell apparatuses,
relays must be employed, because they require a very considerable development of magnetic force, in order to produce a sufficient sound to be distinguishable.
In America, bells are wholly unnecessary on the Morse
Electro-Magnetic Telegraph lines. They are serviceable on the
House, Hughes, Barnes, and other printing apparatuses. On
electro-chemical telegraph lines, bells are indispensably necessary. The ordinary relay magnet produces a sound, which, to
the expert, is intelligible. On the German lines, sometimes
bells are employed.
23