, 1875-04-24

that it can be said to rass across air
for all purposes connected with telegraphy
1t may be reckoned impervious. If then we
could run our telegraph wire from one place
70 another through the, air without touching any
thing, we should have the perfection Of O
telegraph line as it is we may have leakage
arising from several d different causes which
allow only q part OF the current to reach its
destination. There lie, firstly, the state of the
air during heavy rain or q continued damp in
the Atmosphere, when some of the current may
escape direct from the wire. This, however, is
not enough to cause serious Inconvenience.
Secondly there are the supports whenever q
telegraph wire is supported on posts these are
provided with a kind of china cup or some
thing answering to it to which the wire 10
fastened. This is to guard against leakages
china being another i~nsulator, but in spite
of precautions there is always liable to be
leakage over these cups, and especially when
they are wet with rain or covered with dust or
smoke. Though the leakage is small at each
place yet where there are a number. as there
must be on a long line, a good deal of the
current is lost in this way. But with care and
attention much of this may be obviated.
More serious still is the liability in some places
to have trees or other things fall against the
WAAOB ASA "a S"N VAN OAConoG uVtC 10 VI OOulO04
an end to all signaling, but even if it does,
not, j: is possible that 50 much of the cur
rent may escape from the line at this point
as to render it useless until the cause Of
leakage is removed. In submarine cables
again the air cannot be used in any way,
in fact they were not known until Faraday
in 1916 proposed gutta perCha as an inSulator.
-Four years after the first submarine cable was
laid from Dover to Calais. All telegraph
cables are made in this way, a copper wire with
q covering of gutta percha, or as in the case
of the cables we have here, Of india rubber.
Neither of these substances have anything
like the value of the air as inSulatOrs. For
one thing, it is obvious We can have only q
limited thickness of india rubber, while land
telegraph lines are II feet or more from the
ground, but even for equal thicknesses air
would be much better. Yet practically very
much more of the current is available on
most submarine than on land lines of equal
length, on account of their not being exposed
to leakage from the various causes We have
mentioned. There is also another way In
which air would. be much superior as an irsula-
tor which is more difficult to explain suffice it
10 say that a coating of gutta percha or india
rubber retardS the passage of the current but
however long an serial telegraph line may be,
no appreciable interval elapSeS between send
ing and receiving a current, while on the Allan
tic cables where the i~nsulator is gutta percha
quite 2 tenths of a second is said TO elapse
between sending a current and the least indioa-
ton being received iu Newfoundlt nd, while 1t
is about 3 seconds after q current is put ON
before the lull amount can be received on
such a line. This is o serious Inconvenience and
in the case of the Atlantic cable has given
rise to the expence of a new cable being laid
to relieve the traffic on the older onss; the
retardatiOn however is very slight on short
Illness
We have then our source of electricity and
the means Of transmitting It to a dis
lance, that is the battery and the line, bus
one feature may here be noticed when We get
our Gu, rent OF electricity ana are able to convey
it a number of miles We produce a certain
pignal. but when this is done we bave not got
rid of it. To explain this, suppose we take as
on illustration: From q reservoir of water We
pump some through a pipe to a distance whore
it turns a wheel, but if beyond the wheel
the pipe 10 closed, water will soon cease to
fow, and even if it could flow, after a while
our reservoir would be drained unless we
provide for a fresh supply of water. We must
therefore do one Of two things we can take
a second pipe back from the wheel to the
reservoir, or we can let the water run out at
the fer end let us say, it is into the sea; then
By establishing a connection between our reser.
voir and the sea, water will flow as long az
we continue to pump, and We do away with
the need of return pipe. 1t is precisely the