, 1877-11-03
TELEGRAPHINC.
HOW MESSAGES ARE TRANSMITTED orEn nm
ATLANTIC cARIR

| ATLANTIC CABLE
In overland lines the current trayerses
the wire Suddenly, Ike bullet, and At its
ful! strength, so that if the current be suf;.
ciently strong the instruments will be work.
ell at once and no time lost But It IL quite
different in submarine cables There tho
current is slow and varying. 1t travels
along the copper wire In the form Of . wavs
or unduIa.ioa, and is received feebly 1t 6rst
then gradually rising to its maximan q
strength, and finally dying away ag,iA -
slowly as it rose. In the French Atlantic
cable no current can be detected by the most
delicate galvanoscope at America for the Ervt
tenth of second after it has been put on aA
BreSt: and it takes about half a second for
| the received current to reach its maxinuM
value. This IS owing to the phenomena Of
| induction, very important in submarine cab
| les, but almost entirely absent in land lines
In submarine cables rs is we'll known, the
| copper wire which conveys the current is in
sniated from the sea water by an envelope
usually Of gut.a percha. Now the electrici-
| ty sent into this wire induces electricity Of
al opposite kind TO itself IN the sea water
/ outside, and the attractions set up between
! these two kinds '-hoidS back the curreat
1 In The wire and retar1s its passage to the 70,
celving station.
! 1t follows that with receiving instrument
| set to indicate particular strength Of cur
rent the rate Of signalling would be very
| slow on long lines, compared with land lines,
and that different form of instrument Is ne
qaircd fer cable work. This fact stood great
Il III the Nay of carly cable enterprise. Sir
| William then Prcfessor) Thompson first
solved the diiiicaity by his invention of the
mirror galvanometer, and rendered at the
same time the Atlantic Cable Company a
commercial success. The merit of this ~
ceiring instrument is that it indicates with
extreme scnsiblity all the variations of tho
current in the cable; SO that. instead of hav-
ing to wait until each signal wave sent into
the cable has travelled to the receiving end
before sending another series at waves may
be sent after each other IN rapid succession.
These waves, eneroaching upon each other
will coalescc at their bases; but if the crests
remain separate the delicate decipherer at
the other end viii take ccgnizance OF them
and make them known to the eyes a. the
distinct signals Of the message
The mirror galvanometor Is at once beau
tifui!y simple and eXquisitely scientific. 1t
| consists OF very long coil of silk-coyered
copper wire, a:i IN the heart Of the coil
within little air chamber, a small round
mirror, having four tiny magnets cemeated
to its back, Is hung by single fibre OF f9ss
silk no thicker than a spiders line. The
mirror Is of film glass silyered, the magnets
of hairspring, and both together sometimes
weigh oniy one-tenth Of Il grain. A beaEa ct
light IS thrown from lamp upon the mirror
and reflected by it upon a white screen or
scale. :, few feet distant. where it froms .
bright spot of light. When there is no cur.
rent Oh the instrument the spot of hight re.
MAINS stationary AZ the zero portion OF the
screen; bat the instant 4 current traverses
the long wire Of .hc coil the suspended mag
nets twist themselves horizontally out Of
their former position, the mirror is, Of course.
inclined with them, and the beam OF hight Is
reelected along the screen to one side or the
other according to the nature of the current
-tha: is to say, a current from the copper
pole of the ba:tery-gives detection to the
right of zero, a negative current or current
from the zinc pole of the battery will give
detection to the left of zero, and rice rersa.
The AIR in the little chamber surrounding
the mirror is compressed at viii, so as to act
like a cushion and "deaden" the movements
of the mirror. The needle is thus prevented
from idly swinging about at each detection,
and the separate signals are rendered abrupt
and dead beat" rs it is called At re-
ceiving station the current coming IN from
the cable has simply to be passed through
the cell of the speaker" before 1t IS sent
into the ground, and the wandering light
spot in the screen faithfully represents al its
variations to the clerk, who, looking ON, IN
terprets these and cries out the nlessage word
by word. The small weight of the mirror and
magnets which form the moving part OF the
instrument, and the range to watch the MN.
ute motions Of the mirror can be magnlfied
on the screen by the reflected beam OF hight
which acts as long, impalpable hand or
pointer, rendering the mirror galvanometer
marveIlousiy sensitive to the current, espe.
ciallv when compared with other forms OF
receiving Instruments. Messages have been
sen. from England to America through ono
Atlantic cable and hack again to England
through another, and there received on the
mirror galvanometer, the electric used being
that from toy battery rnade out Of lady:
silver thimble. grain Of zinc and a drop OF
acidulated water.