At Fig. 80 is represented a diagram of such an apparatus. _a_ is an armature working on a pivot between the two horns of an electro magnet _b b_, and held in position by a spiral spring _c_; the latter is in connection with a regulating screw, by which more or less pressure may be brought to bear in an opposite direction to that of the attractive action of the electro magnet. A stud _i_ regulates the distance to which the armature may be drawn back; _d_ is a shutter on which a reference number for each mine should be indicated, attached to a lever pivoted at the point _e_, the inner arm of which is just long enough to catch under the point of the armature _a_; when a current of sufficient strength is passed through the coils _b b_ of the electro magnet, the armature _a_ is attracted, releasing the lever attached to the shutter _d_, which by its own weight falls into the position shown by the dotted lines. _f_ and _g_ are two mercury cups, the former being in connection with the signalling current, and the latter with the firing current. When the lever is horizontal and the shutter drawn up and ready for action, the circuit of the signalling battery _s_ is completed through the mercury cup _f_, along an arm _h_ of the lever to the pivot _e_, and thence to the mine by the line wire _w_. When the circuit closer is struck by a passing vessel, and consequently the shutter thrown into the position shown by the dotted lines, another arm _k_, a prolongation of the lever, falls into the mercury cup _g_, which latter is in connection with the firing battery _F_. The armature _a_ is prevented from coming into actual contact with the horns of the electro magnet by two small studs. The object of this is to prevent any effect of residual magnetism which might otherwise interfere with the rapidity of action of the armature when released and drawn back by the spring _c_. [Illustration: FIRING KEYS, SHUTTER APPARATUS. PLATE XX] _The object of employing Mercury Cups._--Mercury cups were devised in the place of the springs used in connection with the original design of a shutter apparatus, for the reason that electrical circuits dependent on the pressure of springs are always liable to interruption from dirt or oxide intervening between the points of contact. _Shutter Apparatus used with a Circuit Breaker._--When the circuit breaking system is used with the shutter signalling apparatus, the action of the armature in releasing the lever must be reversed; that is to say, that when the current is passing and the armature _a_ attracted to the electro magnet _b b_, the shutter _d_ must be held up, and when the current ceases, and the armature _a_ drawn back by the spring _c_, the lever must be released, and the shutter allowed to fall. This is effected by altering the end of the lever, so that it hooks into, instead of abutting against the armature _a_.