546 
POPUIAE SCIENCE EEVIEW. 
to a greater or a less extent_, with the primary current ; and 
where great lengtjis of cable are concerned it becomes an 
element of serious consideration. If a coated wire is placed 
on the earthy and an electrical current is established in it, the 
earth acts the part of the second wire and induction is developed 
on its surface. Mr. S. A. Yarley has examined this problem 
with much care^ and has arrived at the following conclusions. 
In a suspended wire the insulatory medium of the air takes the 
place of the gutta-percha of the submarine circuit. The earth, 
which is the nearest conductor, is a considerable distance off, 
and is only on one side of the wire ; therefore but little induc- 
tion can take place between the wire and the earth. Never- 
theless, induction to a certain extent does take place, and it 
can be detected with delicate apparatus in circuits of very 
moderate lengths. If the distance between the wire and the 
earth is decreased, induction will be developed more strongly, 
and the wire could be brought down, step by step, until the 
condition of a submarine circuit would be approached, where the 
earth surrounds the wire on all sides, and is only separated from 
it by the thickness of one-eighth or three-sixteenths of an inch 
of gutta-percha, a substance possessing, moreover, specifically 
a much greater inductive capacity than air. It, therefore, 
appears that the conditions are precisely the same, only differ- 
ing in degree. It is, nevertheless, evident from this, that a 
submarine wire is placed under circumstances of greater diffi- 
culty than those which surround an aerial wire. This difficulty 
increases with the length of the wire, retardation becoming 
more and more powerful as the distance through which the 
wave has to move is extended. A submarine cable may be 
regarded as a Leyden jar ; and the telegraphic indications 
are analogous to the discharges of a coated glass. An im- 
pulse is given by making a connection with a voltaic battery 
at one end of a wire suspended in air — and it indicates 
telegraphically by producing magnetic disturbance at the 
other — and a succession of impulses will rapidly give a suc- 
cession of indications. Now, in an insulated submarine wire 
this is not exactly what takes place. A distinction has to be 
drawn between the simple arrival of a current — which may be 
regarded as instantaneous — and the production of a telegra- 
phic signal. After having charged the wires sufficiently to 
develop an appreciable current — owing to the wire taking 
some time to empty itself — if currents are sent in succession 
with any rapidity, they will blend into one another, and, 
instead of getting a series of distinct impulses at the further 
extremity, a continuous undulating wave will be obtained. 
This sluggishness has been obviated, to a great extent, 
in circuits of moderate length, by employing opposite 
