, 1875-04-24

u SUFFICIENT 10 COuu6Cb 10 Tue OUTSIDE OF the
cable itself, the iron s~heathiog of which being
10 contact with the sea forms az efficient earth
connection. Outside our telegraph Office here in
Christiansted an attempt was made TO secure this
ear!h connection by burying a copper plate
sbis did not answer well, through the ground
being dry, SO a wire has been carried direct 10
the water.
The remaining thing to be noticed now
that we have secured flow Of electricity
80 the distant station is in what way it shall be
used to produce signals. 10 the first telegraphs
Shry used what is called a needle instrument,
which is simply a convenient form Of lie gall
vanometer which had been invente. me
years before, being simply the Sept #
Orsted's great discovery that a current Of et-
trlcl'y tends to move a magnetised needle
suspended near it in fact all telegraphic zig
nails are produced by the Influence of the cur
rent on a magnet. 1t was found that, while a
single wire conveying a current would move
a magnetic needle to some extent if it were
brought back again on the other side of the
needle, it would tend to move it still more
again the wire could be brought round and
lie side by side with the first part with ad-
vantage, provided the parts did not touch it
could further be taken round back again and
so ons each successive turn adding to the eff
Z L y ~ q OIldi tAlelG' 4 MS Soon D6caU8
the galvaDometer which consists simply of o
coil of wire covered with silk to prevent touch
ing, in the middle of which is a magnetised
needle on a current being sent through the
instrument in Que direction the needle moves
round, let us say to the right, a certain dis
lance; when the current ceases the needle
takes up It's original position if the current
is sent through the instrument 10 the reverse
direction the needle moves to the left. Il 10
easy to See that by arranging and combining
Shese movements an al pkabet can be formed,
thus the letter K so much in use in the English
IangSage and indeed in many others is now
represented by a siogle signal, while the letter
L takes four, two movements Of the needle to
the righ: followed by two TO the left denoting
that letter. In a. decide instrument all that
is visible is a dial with a needle moving 10
front Of it this needle is connected by a spin
die with the needle in the middle Of the coil
which lies Immediately at the back of the
64ar =l0 what lis known as a mirror iDstru-
ment, where the sigcals consist of the move
ments of Il spot of light, the arrangement
consists in having enc or more magnets fixed
ON the back OF a small mirror which moves as
they are detected and which, receiving a beam
of light through u small hole, reflects it on a
screen placed suitably for the purpose. In
she Morse instrument, which records its sig
malls, there is a somewhat different arrangement;
here the current passes round two fixed bars
of iron, thus forming an electromagnetic this
attracts a small pIvoted armature the other
end Of which at the same instant strikes a roll
of paper moved by clockwork, in doing which
it marks it. In this instrument tbe 4 %tiuction
is made by using long aud short ST a line
being traced on the paper al the tins 81 cer nt
is continued. This instrument is u, or
working the line from here to St. Tho 4s.
The only essential part now which we have
not noticed is the apparatus for connect
ing the battery with the wire, this is called a
key and is simply a mechanical arrangement
for making instantaneous contacts, a function
very necessary for the practical working of
the line, as will be seen when it is remembered
that clerks have succeeded in sending 40 words
in . minute, whore each word may be taken
10 consist of 5 letters, and each letter as
We have seen from one to four signals. If
we suppose two signals to each letter that will
make 400 signals in the space of one mute.
When it is remembered that this means 400
distinct movements of the hand in that time,
it will be seen that a good deal of practice ID
necessary to produce expert sending.
=. COOKE.