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_, _m_^{1}. [Illustration: M^{c.}EVOY'S MAGNETO ELECTRO CIRCUIT CLOSER. PLATE XV] The connections of the wires are made as follows:-- The line wire _w_ is led through the base of the apparatus and connected to a piece of brass under the ebonite support _l_, in connection with one of the wires of the bobbin _m_, the other wire of which is attached to the metal strip _s_; the wires of the bobbin _m_^{1} are connected, the one to the metal strip _s__{1}, the other to a piece of brass under the ebonite support _l_; from this latter piece of brass a wire _w__{1} is led to the brass screw _x_. The wires _w__{2}, _w__{3}, from the fuzes are led, the one to the brass screw _x_, the other to a screw _y_, which forms through the metal of the apparatus the earth plate. One of the wires of the telephone _t_ is connected to the brass screw _x_, the other _w__{4} is connected to the piece of brass to which the line wire _w_ is also attached. While the circuit closer remains in a state of rest, the current from the signalling battery flows along the line wire _w_, up the telephone wire _w__{4}, through the telephone which has a high resistance, then by the wire _w__{2} through the fuzes, and to earth by the wire _w__{3}. On the circuit closer being struck, by which cause the armature _n_ is brought up to the cores of the bobbins _m_, _m_^{1}, and the piece of brass _r_ in contact with the metal strips _s_, _s__{1}, the signalling current, instead of circulating through the high resistance of the telephone _t_, passes round the bobbin _m_, down the metal strip _s_, across the brass piece _r_, up the metal strip _s__{1}, round the bobbin _m__{1} (thus forming an electro magnet of _m_, _m__{1}), and by the wire _w_, direct through the fuzes to earth, and so explodes the torpedo. The effect of the telephone resistance being cut out, is to strengthen the signalling current, and enable it to work the shutter apparatus and so throw the firing battery in circuit and explode the mine. The advantages of this circuit closing apparatus are:-- 1.--Simplicity. 2.--Compactness. 3.--Increased certainty of action, due to the sustained contact of the armature _n_, on the apparatus being struck. 4.--Additional means of testing a system of electrical submarine mines, which is afforded by the telephone:-- When this form of circuit closer is put in action by a friendly vessel coming in contact with it, or when experiments are being made, the signalling current must be reversed, so that no doubt may exist as to the armature _n_ having dropped, on the apparatus coming to rest.