2.—A single armoured cable to be connected as in foregoing instance.
3.—A T junction box for the branch system of electrical contact mines.
Junction Box for Multiple Cables.—At [Fig. 43] is represented a plan of lower half of this form of junction box. It consists of a pair of cast iron plates of precisely similar form to the one shown at [Fig. 43], and so made as to be capable of being fastened tightly together by means of four bolts and nuts passing through the holes a, a. The grooves b, b at the two extremities are just large enough to grip the armoured cable firmly, when the upper and lower parts are screwed together. A larger space is provided in the hollow for the joint.
Junction Box for Single Cored Cables.—For this purpose a junction box similar to, but smaller than the one above described is employed.
T Junction Box.—This form of junction box is employed when the system of electrical contact mines on branches from a single cable is used. This system is dependent on the use of a platinum wire fuze in connection with a platinum wire bridge in each branch close to its junction with the main cable.
This form of junction box, which is shown at [Fig. 44] is very similar to the one used for the connection of two multiple cables, only differing in its shape, which is that of a T. a is a disconnector, which will be described further on; b, b, b' are the armoured electric cables, b, b being the main, and b' the branch cable in connection with the forked joint formed within the T junction box; c, c, c are Turk's heads formed to prevent any strain being brought on the forked joint. This form of Turk's head is made by turning back the wires of the cable armouring, and frapping them round with spun yarn until the necessary size and shape is attained.
McEvoy's Turk's Head.—Another form of Turk's head, devised by Captain McEvoy, is shown at [Fig. 45]. It consists of two separate pieces of brass, a and b, the former screwing over the latter. The mode of using it is as follows:—
Slip the piece of brass b over the cable c, and turn back the wires of the cable d, d, &c., so that they lie against the shoulder of the brass piece b, then slip the other piece of brass a over the cable and screw it on the piece b, firmly jamming the turned back wires d, d, &c. This is a very neat and quick method of forming a Turk's head, and it should be invariably used in preference to the foregoing method, which is clumsy, and which takes some time to form.
The section of a disconnector is shown at [Fig. 46]. It consists of an iron cover, or dome a, which is provided with a screw fitting on to another screw on the ebonite body b of the apparatus. When the dome a is screwed tightly down on the washer i, the whole is made perfectly watertight. c, c are insulated terminals for connecting the cores of the branch and main cables after their armouring has been removed, as shown at [Fig. 44]. d, d are two copper conducting wires (No. 16 B. W. G.) passing through the centre of the ebonite body b, and projecting into the interior of the apparatus. These wires are held in position and insulated by means of a composition formed of a mixture of pitch, tallow, beeswax and gutta percha. This composition is put on whilst hot and allowed to cool gradually, when it becomes hard and durable. Great care is necessary to ensure the cavity within the ebonite body b being completely filled, as otherwise a leakage might occur, owing to the great pressure of water at depths where the disconnection would be generally used. f is a boxwood cover which is slipped on, and fits fairly tight to the ebonite body b; g is a piece of thin platinum wire, weighing 1·6 grains to the yard, and being 4/10 inch in length; h is an ebonite pin, which passes through two small holes in the boxwood cover f, into which it fits tightly, and in such a position as to be directly beneath the platinum wire bridge g, when the boxwood cover f is fixed on. The pin h is pushed through the holes in the cover f from the outside, so as to pass beneath the bridge g after the priming has been inserted, and the cover has been placed on.