The Sun.
London, England, 1867-02-18 00:00:00

Royal Institution.

The lecture theatre of the Royal Institution was
crowded on Friday evening to hear a discourse by
Mr. C. F. Varley on the Atlantic telegraph. On
the table there were displayed coils of the cable
successfully laid last year, and of the two cables
which had been picked up from a depth of two
miles, one of them having lain at the bottom of the
sea for two years Mr. Varley did not enter into
the consideration of the mechanical construction of
the cable, nor of the plan of laying it down ; neither
did he state any of the practical results of the work-
ing the Atlintic telegr ; but he confined him-
self to the scientific part of the subject, and to the
explanation of the difficulties which arise in attempt-
ing to transmit electric signals through great lengths
of submerged insulated wire. He observed that
the transmission of electricity is not governed by the
same uniform laws as the transmission of light and
radiant heat, but that the rapidity with which it is
transmitted depends on various conditions. The
intensity of the force, or, as it is now frequently
called, the " potential" of the electricity, the thick-
ness, the length, and the condition of the conduct-
ing wire have great influence on the rapidity with
which signals can be transmitted through a sub-
marine telegraph cable. As a practical indication
of the force that may be brought to bear for that
purpose, Mr. Varley had a voltaic battery of 800
cells, the discharge of which perforated holes in tin- '
foil as large as a sixpence, and which might have
produced dangerous, if not fatal, consequences with-
out great caution. The remarkable effect of the
retention of an electric charge by insulated sub-
marine wires, discovered by Professor Faraday in
1854,was then explained and illustrated. It was ascer-
tained by Professor Faraday that when a gre,at length
of wire, covered with gutta percha, is immersed in
water and charged with electricity by sending a tele-
graph signal through it, the wire retains the charge
for a short time, so as to prevent a second signal from
being transmitted until the wire is discharged. The
insulated wire when in water is in the same condi-
tion as a Leyden jar; the wire being the inner
metallic coating, the gutta percha takes the part of
the glass, and the water serves as the outer coating
of the elongated Leyden jar. It is well known that
in discharging an ordinary Leyden jar a portion of
the charge is retained, and the same effect takes
place to a much greater extent in the insulated sub-
merged telegraph cable. This has been imitated by
employing an apparatus called a condenser, by which
the electric current is retarded by passing through
water and through a considerable length of very
fine wire, the resistance of which to the tranmis-
sion of electricity is ascertained to be equal to the '
resistance offered by a given length of telegraph
wire. Mr. Varley exhibited a series of condensers
of this kind, calculated to show the amount of de-
tention of the electricity at various stations extend-
ing from England to Australia. The effect was
first shown on the Atlantic telegraph cable between
Valentia and Newfoundland. When a signal was
sent through the resistance coils only, it was almost
instantaneously perceived at the representation of
the Newfoundland station, in the same manner as
if a wire were suspended in air between the two sta-
tions ; but when the condenser was put in action
an interval of some seconds elapsed before the signal
was visible at the Newfoundland station. Mr. Var-
ley then proceeded to describe the contrivances in-
vented by Sir W. Thompson and himself to facili-
tate the discharge of the electricity in the cable, and
to accelerate the transmission of biguals. They con-
sisted essentially in partially clearing the wire by
connecting one end of it with the earth, and by
charging it with currents of negative electricity at
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When
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cable was illustrated in a very interesting manner.
There was thrown on a screen a series of 'images
in a perpendicular line, reflected from as many of
Sir W. Thornscn's minute reflecting galvanometers.
Each galvanometer ss
supposed station of a telegraphic route between
England and Australia : one bring Gibraltar,
ia
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o Australia, Rangoon
andot n
sig-
nal was transmitted the image representing Gib-
raltar darted off quickly to the left-hand side of
the screen, Malta moved soon after, and each one
followed in succession more and more slowly until
Australia was reached ; but before that occurred
the wire at Gibraltar had become nearly discharged
and the image returned towards its original posi-
tion. The effect of transmitting currents of nega-
tive electricity was then exibited. The image of
Gibraltar darted off rapidly towards the right, while
the images of the more distant stations continued
to move towards the left, and before the time the
Australian image moved the line was nearly cleat