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. On a brass plug being put in the hole _l_, the signalling current will flow to the plate _k_, thence through the lower and upper bobbin to the spring of the armature, along the latter to the shutter lever, and from the pivot through the main line wire to the mine. The innermost brass plates _H H_ are all connected in the same metallic circuit, and to them are attached by means of the binding screw _D_ the test battery and galvanometer. Thus on the brass plug being removed from _l_, and placed in _m_, the signalling battery is cut out of circuit, and the test battery thrown in. In this way the condition of each individual mine may be ascertained while the connections of the remaining mines are left undisturbed. The positive pole of the firing battery (the negative being to earth) is connected to the terminal _S_ at the right-hand corner of the lower ledge of the box; the plate to which the terminal _S_ is fixed is divided at _G_, the left-hand portion being connected to a bar which runs horizontally the whole length of the box, and in metallic connection with each mercury cup _g_, Fig. 80. A brass plug is placed in the hole _G_, and when from any cause the lever drops, the firing battery will be thrown into circuit, and the mine to which the lever that has fallen is attached will be exploded. _Shutter Instrument and Observing Telescope._--Each mine is given a number, which is put on the disc of the shutter instrument connected to it, and also on the corresponding tablet _C_. From the brass plate in connection with the spring _c_, Fig. 80, a wire is taken to the terminal _f_, Fig. 81, on top of the box. From this terminal a wire is led to the connections of the observing telescope, and thus the mines can be fired by judgment if required, without the aid of the circuit closer.