152 
THE TELEGRAPH. 
. for the longitudinal strips of india-rubber are previously mixed 
with 6 per cent, of sulphur and 10 per cent, of sulphide of 
lead. 
The bands wound spirally round the conductor are composed 
of india-rubber containing the separator, that is to say, of 
25 per cent, of oxide of iron, which acts by preventing the 
sulphur from attacking the conductor. This plan of insulation 
by india-rubber is the invention of Mr. Hooper, and it has 
given good results especially in tropical seas. 
The relative value of india-rubber and of gutta-percha has 
great interest from a practical point of view. Gutta-percha 
has been much studied since 1851, as regards its electrical and 
plastic properties, and at the present day the air-bubbles and 
impurities that originally impeded its use can be completely 
obviated. As gutta-percha becomes semi-solid at a comparatively 
low temperature, its use in warm climates where it is exposed 
to the action of a burning sun, is, of course, attended with 
much risk. Gutta-percha joints are also liable to soon become 
damaged, unless they have been made with the greatest care 
by skilled workmen. On the other hand, gutta-percha pos¬ 
sesses the most invaluable properties; it is not affected by sea¬ 
water, and this is a quality that cannot be too highly appre¬ 
ciated; its electrical properties are such that the current 
received at one end of a gutta-percha cable, however long it 
may be, loses less than 1 per cent, of its initial power. The 
actually existing cables certainly approach the possible limits 
of perfection as regards their electrical qualities, but this does 
not prevent further improvements from being sought for. 
Mr. Willoughby Smith, the electrical engineer of the gutta¬ 
percha works of the Telegraph Construction and Maintenance 
Company, has succeeded in making a particular kind of gutta¬ 
percha that nearly equals india-rubber as far a regards its low 
specific induction. We must remember that cables, acting as 
so many huge Leyden jars, exhibit induction phenomena that 
sensibly retard the transmitted currents; india-rubber possesses 
in a less degree than gutta-percha this inductive capacity, and 
in this respect it is superior to the latter. Mr. Smith’s new 
gutta-percha and Hooper’s india-rubber have nearly the same 
electro-static capacity, the ratio between being 98 : 100. 
India-rubber is less liable to be spoiled in working than 
