Telegraph, Dial.
A telegraph in which as receiver a dial instrument is used. A pointer or
index hand moves around a dial. The dial is marked with letters of the
alphabet. The movements of the pointer are controlled by the
transmitting operator at a distant station. He by the same actions moves
a pointer on a duplicate instrument before him and the two are
synchronized to give identical indications. Thus a message is spelled
out letter by letter on both dials simultaneously. The motions of the
index are generally produced by what is virtually a recoil escapement.
The scape wheel is carried by the axle of the index, and a pallet or
anchor is vibrated by an electro-magnet whose armature is attached to
the stem of the pallet. As the pallet is vibrated it turns the wheel and
index one tooth for each single movement. There are as many teeth in the
wheel as there are characters on the dial. The two instruments being in
duplicate and synchronized, the pallets move exactly in unison, so that
identical readings of the dials are given. The pallets may be moved by
any kind of make and break mechanism, such as an ordinary telegraph key.
The index moves by steps or jerks, so that the system is sometimes
called step-by-step telegraphy.
Fig. 312. DIAL TELEGRAPH.
In the cut the make and break transmitter is shown at v v, with its
handle and contacts g and t. This mechanism sends impulses of current by
F and Z to the receiving magnet l. This attracts and releases its
armature K from contact into the position indicated by the dotted lines.
This works the rocker n on the pin o, and actuates the double or anchor
pawl s r, which turns the pallet or scrape wheel m.
The system is dropping into disuse, being supplanted by the telephone.
Synonym--Step-by-step Telegraph.