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. In practice precautions must be taken to prevent the copper tape covering from being brought into contact with any iron, for were such to happen, electrical action would at once ensue, causing the iron to corrode with enormous rapidity. In some of Siemens's cables, vulcanised india rubber replaces the gutta percha insulation. Iron covered cables, either galvanised or plain, are manufactured as well as the copper tape covered ones by that firm. _Hooper's Cable._--This form of cable is represented at Fig. 31. It consists of a metal conducting wire, generally copper, _a_, covered with an alloy to protect it from chemical action, the insulating substance _b_, known as Hooper's material, previously described at page 39, a covering of tarred hemp _c_, and an outer covering of iron wires (No. 11 B. W. G.), each of which is separately covered with tarred hemp and wound on spirally, _d_. Gray's cable is very similar to the one just described, the chief difference in it as compared with Hooper's being the absence of the separator.