It is placed in the mine in such a manner that when undisturbed it maintains an approximately upright position.
It consists of a metal tube a into which the cup b of vulcanite, or other insulating material is fixed. The cup is contracted at some distance from the top by the perforated plug c, which is also of insulating material; d is a metal pin fixed into the bottom of the cup b, it is connected with the wire e, which is insulated and passes to the battery; f is a metal plug closing the tube a and the cup b at the top; g is a wire attached to the plug f, and passing from it to an earth connection. The cup b is filled with mercury up to the level of the plug c. By the contact of a passing vessel the instrument would be tilted sufficiently to cause the mercury to flow into contact with the metal plug f, thus completing the electric circuit and exploding the mine.
This form of circuit closer, though not generally adopted, would, on account of its being less liable to derangement by the motion of the waves, or by the explosion of an adjacent or counter mine, seem to fulfil the many requirements of a circuit closer for general service.
McEvoy's Weight Magneto Circuit Closer.—This form of circuit closer, which is shown in section and plan at [Fig. 60] and [61], is one of the most important improvements that has ever been effected in such apparatus, and bids fair to become universally adopted.
A heavy metal conical shaped weight a (Fig. 60), hollowed out in its base and working in a ball and socket joint b, rests on a solid brass base c, and is so arranged that on the apparatus being struck, the weight a will fall over, pivoting on one of its supports d, d; e is a band of india rubber, encircling the weight a, for the purpose of preventing a jar on its falling against the sides of the brass cylinder f, which contains the weight a and joint b. A brass rod g, connected to the ball and socket joint, passes through the base c, through a strong spiral spring h (which latter rests on an adjusting screw k), through a piece of ebonite l, which supports the bobbins and core m, m1; then between these bobbins m, m1 through an armature n, which is pivoted at p; and lastly through a slight spiral spring o, which is kept in position by the adjusting screw i.
The armature n is fitted with a small piece of brass r, so arranged that when it (the armature) is in the position shown in [Fig. 60], this piece of brass r does not make contact with the two strips of metal, s, s, between which it, r, works; but when the armature n is in contact with the cores of the bobbins m, m1, then the piece of brass r makes contact with the metal strips s s, and so makes a short circuit for the electric current. An ordinary telephone t, [Fig. 61], in which some small shot, bells, &c., are placed, is fixed to the top of the brass cylinder f.
Action of Circuit Closer.—The action of this apparatus is as follows:—
On the mine carrying this form of circuit closer being struck by a passing vessel, the weight a is caused to fall over towards the side of the brass cylinder f, thus allowing the strong spiral spring h to act on the brass rod g in an upward direction, by which means the armature n is brought into contact with the soft iron cores of the bobbins m, m1.
Mc.EVOY'S MAGNETO ELECTRO CIRCUIT CLOSER.