
May 4, 1871] 

NATURE 9 




and the vast system of inter-oceanic communication that 
connects the civilised world together, has been framed. 
At the date of Wheatstone’s first experiment, gutta- 
_ percha was undiscovered, and its insulating power un- 
known. By the employment of this gum, the electrical 
_ condition of the submarine cable, up to a certain standard, 
has been under ordinary circumstances rendered secure. 
Such being the case, and for the purpose of comparison 
_ hereafter, it is well to examine a little into the properties 
of this gum and that of india-rubber, another vegetable 
substance possessing insulating properties of the most 
_ remarkable kind, as applied to the construction of sub- 
_ marine cables. . Gutta-percha, as is well known, is a vege- 
table gum, which becomes plastic and soft at a com- 
paratively low temperature, about 100° F. Subjecting 
the gum to repeated cleansing processes to free it from 
impurities and extraneous vegetable matter, it is rendered 
tolerably dense and homogeneous, and in this state it is 
applied in successive layers or coats round the copper 
conducting wire as the insulating material, forming the 
“core” of the submarine cable, which is then termed “ in- 
sulated,” that is, capable to a certain extent of preventing 
the lateral escape of any electric current or charge which 
may be passed into the wire. A short investigation is now 
necessary to be made of some of the circumstances which 
take place when a wire thus insulated is submerged and 
subjected to the charge of an electric current. If the 
wire were absolutely insulated, that is, if gutta-percha 
__were a perfect insulator offering an indefinite resistance to 
_ the passage of the current through its substance, any 
given quantity of electricity passed into the wire would 
remain there for a given time without loss, in the same 
way as when water is poured into a vessel, the level re- 
mains intact so long as there is no leakage. The amount 
of this leakage through the gutta-percha, or, in other 
words, its “ conductive resistance,” determines the insu- 
lating power of the cable. But this is not all that has 
to be considered ; other circumstances affecting the value 
of the insulation come into play. The following analogous 
example will explain. When a leech is allowed to crawl 
through a glass tube, the head and body pass out first, 
while the tail—long and attenuated—is slowly withdrawn. 
So with the passing of an electric current through an in- 
sulated conductor, a portion of the current lags sluggishly 
behind, absorbed, as it were, into the substance of the 
insulating medium, and taking time to discharge itself in 
proportion to the amount of the sucking up, or “ inductive 
capacity ” of the insulator, for, in this respect, both gutta- 
percha and india-rubber may be regarded as a sponge, 
the current penetrating into the pores of the substance. 
Without entering further into detail regarding the laws 
regulating the transmission of the current, it is sufficient 
to remember that the speed or power of transmitting a 
given number of messages in a given time over any cable 
depends materially upon the proportionate values of the 
“conductive resistance” and “ inductive capacity” of the 
insulation. Thus there is at once established a measure by 
which the value of all known insulating materials may be 
determined and compared together, that is to say, if two 
cables of equal length and similar construction are 
taken—the one insulated with gutta-percha, and the 
other with india-rubber (Hooper’s india-rubber)—the 
relative value and working speed of each can be 
accurately determined and compared. The successful 
employment of india-rubber as an insulating medium for 
submarine cables is of more recent date, and the estima- 
tion in which it is now held for that purpose is entirely 
due to the beautiful process emplayed in its manipulation 
by Mr. W. Hooper, of Mitcham. It is well known that 
india-rubber possesses a much higher insulating power 
than gutta-percha ; as a gum it is also denser, more ho- 
mogeneous, and infinitely more pliable and elastic than 
gutta-percha, while it is not affected in any considerable 
degree by variation of temperature—all qualities of the 

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greatest importance as connected with submarine cable 
insulation. 
Before entering upon a comparative statement of 
the insulation and speed of gutta-percha and Hooper’s 
insulation, a short notice of the mode by which this 
insulating material is manipulated will be interesting, and 
will serve to give value to the practical data hereafter 
stated. The copper conductor, afier being tinned, is 
coated with an insulation of pure india-rubber applied in 
the shape of a ribbon, lapped spirally round it. Next, 
two strips (one laid above and the other below) of india- 
rubber, chemically prepared to resist the action of sulphur, 
and called the “separator,” are applied so as to completely 
surround the first rubber covering, as it were with a tube ; 
a pair of grooved die-wheels giving the contour, and at 
the same time regulating accurately the guage of the 
core. Exterior strips are then similarly applied of a com- 
pound of rubber and a small percentage of sulphur. The 
whole is then lapped round with water-proof felt tape, and 
exposed for some hours in an oven to a heat of about 
383° F. By this process the three successive coatings 
are welded into one solid, dense, homogeneous mass, hav- 
ing its distinctive features preserved as regards the indi- 
vidual character of the several layers. Thus the heat, in 
driving off the sulphur from the outside coating, has con- 
verted that envelope into an indestructible vulcanised 
rubber jacket. The second layer, or “ separator,” has inter- 
cepted the passing of the sulphur by reason of its chemical 
properties, while at the same time it has allowed an infinite- 
simal trace of the sulpbur to combine with the internal coat- 
ing of pure rubber round the conducting wire, sufficient to 
change its character into an indestructible and non- 
liquifying material, without its becoming in any way 
vulcanised. It is by this beautiful chemical affinity 
between the several layers, each performing its special 
part towards the production of one individual whole, that 
the “Hooper insulation” has succeeded in establishing 
the durability of the preparation, the comparative value 
of which, as compared with that of gutta-percha, will now 
be given. 
First as regards temperature—it has been already 
stated that gutta-percha became plastic at about 100° F. 
At this temperature it loses also almost entirely its insu- 
lating properties; that is to say, if at a temperature 
of 32° F. the insulation of gutta-percha is taken as 
representing 100, at 75° it is reduced to 5°51, or little 
more than a twentieth part, while at the increased tem- 
perature of 100°, its insulating power has further de- 
creased to 1°43, or about one seventieth part. Gutta- 
percha as an insulator is therefore unsuited for hot 
climates, or any exposed position where the temperature 
rises above 70°. Taking now Hooper's india-rubber insula- 
tion at 32° F. to be the same, 100, at 75° we find its insula- 
tion to be 24°50, or about one-fourth part, while at too° it 
is 10°60, or about one-tenth part. Thus at the ordinary 
temperature of 75°, Hooper's core establishes its superior 
insulating properties under temperature in the propor- 
tion of fourtoone. The“ inductive capacity ” of Hooper’s 
core, from its superior density, is only about two-thirds 
that of gutta-percha, while its insulation or resistance of 
the dielectric is fully twenty times greater than that of 
gutta-percha core, as exemplified in the tests given of 
some of the best known cables now at work, 
The following is a list of some of the more important 
cables insulated with Hooper’s core laid up to the present 
time :— 
1. Cable crossing river's in India, laid in 1865, length 46 nauts. 
2. Ceyion Cable, India, laidin 1866 . ey BIN te 
3. India Cable . 7 . 6 2 wares 40 5 
4. Persian Gulf Cable c 6 : Ree wise) ep 
5. Danish-English Cable. . . > 9, 363° 
6. Scotch-Norwegian Cable ' c a TAIN Lean bs 
7. Danish-Norwegian Cable. : : bate, HS; 's5 
8. Orkney and Shetland Islands Cable. ts pees). =, 
