The Shutter Apparatus.—The shutter signalling and firing apparatus was devised to enable the firing battery current to be thrown in circuit without the aid of a personal operator, the signalling current (which is always kept in circuit) at the same instant ringing a bell, by which is known the particular mine that has been struck.
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.
FIRING KEYS, SHUTTER APPARATUS.
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.
To each shutter apparatus an electric bell is fitted, by which notice is given when a circuit closer has been struck. For general service, a box containing seven such shutter signalling and firing apparatus has been adopted, a plan of which is represented at [Figs. 81], [82] and [83]. The connections of the different circuits are as follows:—
The insulated wire of the upper bobbin of the electro magnet is connected to the spring of the armature; the pivot of the lever is connected with the right-hand terminal B, or main line connection on the top of the box; the insulated wire from the lower bobbin is connected to the middle brass plate k in the front ledge of the apparatus, the circuit from B to k being thus completed. The front adjoining brass plate A, provided with a terminal, is connected with the negative pole of the signalling battery, the positive pole being put to earth.