The Hawaiian gazette. [volume] (Honolulu [Oahu, Hawaii]) 1865-1918, 1878-01-16
How Gable Telegraph Lines are
Worked.

In overland lines the current traverses the
wire suddenly. like q bullet, and at its full
strength, so that il the current be sufficiently
strong the instruments will be worked at once
and no time lost. But it is quite different in
submarine cables. There the current is slow
and varying. 1t travels along the copper wire
in the form of wave meditation. and is re =
ceved feeby at first, then gradually rising to its
maximum of strength, aud finally dying away
again as slowly as it rose 10 the French At
lantc cable no current can be detected by the
most delicate galvanoscope at America, for the
first tenth of second after it has been put on
at Brest; and 1t takes about half second for
the received current 10 reach its maximum value.
This is owing to the phenomenon Of Induction,
very important in submarine cables, but almost
entirely absent in land lines In submarine
cables, as is well known, the copper wire which
conveys the current is insulated from the sea-
water by an envelope, usually of gutta percha.
Now. tue electricity sent INTO this wire induces
electricity of an opposite kind to itself IN the
sea-water outside, and the attraction set up be-
tween these two kinds "bolds back" the current
in the wire and retards its passage at the re.
ceiving station. 1t follows that with receiving
instrument set to indicate particular strength
of current, the rate of signalling would be very
slow on long cables compared 10 land lines; and
that different form Of instrument is required
for cable work. This fact stood greatly in the
way OF early cable enterprise. Sir VIM. Thorn
son first solved the difficulty by his invention of
the " mirror galVanometer," and rendered at the
same time the first Atlantic Cable Company a
commercial success. The merit of this receiving
instrument is, that it indicates with extreme
sensiblity all the variations of the current in the
cable, so that, instead of having to wait until
each signal wave sent into the cable has travelled
In ha hafn
series OF waves may be sent after each other in
rapid succession. These waves encroaching up
on each other, will coalesce at their bases; but
| it the crests remain separate the delicate de-
cpherer at the other end will take cognizance
| of them, and make them known TO the eye as the
| distinct signals OF the message. The mirror
| gayanometer is at once beautifully SIMPLE and
exquisitely scientific. 1t consists of long fine
coil of silk-covered copper wire. and in the
heart Of the coil, within a little air chamber
a small round mirror, having four tiny mag
nets, cemented to its back. is hung. by
q single fibre of floss silk no thicker than spl
der's line. The mirror is of film glass, sivered
the magnets of bair-spring. and both together
sometimes weigh only one-tenth of grain. a
beam Of light is thrown from lamp upon The
mirror, and reflected by 1t upon a white screen OIL
scale few feet distant, where it forms brigh
spot OF light. When there is no current on The
instrument, the spot flight remains stationary a
the zero position on the screen but the instan
q current traverses the long wires Of the coil, tht
suspended magnets twist themselves horizontally
out oil their former position the mirror is. o
course, inclined with them, and the beam OF Iigh
is detected along the screen to one side or tn,
other. according to the nature of the current
positive current, that is to say. current frou
the copper pole of the battery it gives deHec
tion to the right of zero. but negative current
or current from the zinc pole of the battery
will give DETECTION to the left of zero, and vic
versa. The air in the little chamber surroundiu.
the mirror is compressed at will so as to act likl
cushion, and -deaden" the movements of th,
mirror. The needle is thus prevented from idll
swinging about at each detection, and the sep
crate signals are rendered abrupt and dead beat,
as it is called. At receiving station the curren
coming 10 from the cable has simply TO be passes
through the coil Of the speaker" before it is sen
into the ground. aud the wandering light spot Oh
The screen faithfully represents all its variation
to the clerk, who, looking on, interprets thes
and cries out the message word by word Tn
small weight of the mirror and magnets whicl
form the moving part OF this instrument and tn
range to which the minute motions Of the miro
can be magni6ed on the screeE by the reheete,
beam Of light. which acts as q long impalpabl
hand or pointer, render the mirror galvanomete