The advantages claimed by Mr. Hooper for this mode of insulating electric submarine cables, are:--
_c._--Capability of withstanding the bad effects of dry heat.
The qualifications essential to a perfect insulated electrical cable for use with submarine mines are as follows:--
1.--Capacity to bear a certain amount of strain without breaking.
2.--Perfect insulation, or at least as nearly so as it is possible to obtain, and composed of a substance capable of being readily stored, and kept for a considerable length of time without being injured.
3.--Pliability so that it may be wound on, or paid out from, a moderately sized drum without injury.
4.--Provided with an external covering capable of protecting the dielectric from injury when used in situations where there is a rocky or shingly bottom, &c.
The insulated wire of a submarine cable is technically spoken of as its _core_.
By a _cable_ is meant to be understood any piece of covered wire.
Several forms of submarine electrical cables have been devised, all of which more or less possess the qualifications enumerated above. The following are some of the most effective:--
1.--Siemens's cable. 2.--Hooper's cable. 3.--Gray's cable. 4.--Service cable.
_Siemens's Cable._--This form of cable is represented at Fig. 30. It consists of a strand _a_, which is composed of three or more copper wires formed by laying up the several single copper wires spirally, several layers of gutta percha, or india rubber, _b_, two coverings of hemp, saturated with Stockholm tar, _c_ and _d_, and several plies of copper tape _e_, wound on, so that each strip overlaps the preceding one, as shown at Fig. 30. The conductivity of the copper employed for the strand is equal to at least 90 per cent. of that of pure copper.
This exterior covering of copper tape is a patent of Messrs. Siemens Brothers, and when once laid down, the cable so covered is very efficiently protected, and of course it is little affected by the action of the sea water. This mode of protection has one great defect, viz., that in the event of a kink occurring in paying out the line, and at the same time a sharp strain being applied, the copper tape at that point is extremely likely to destroy the insulation by being drawn in such a way as to cut through the dielectric. On this account great care must be observed in handling this form of cable.