_Armstrong's System of Electrical Testing._--A very simple method of testing electrical submarine mines, with which low tension fuzes are used, has been devised by Captain Armstrong, R.E., and is shown at Fig. 99. _a_ is the electric cable leading from the shore; _b_ the cable attached to a polarised relay _c_, and connecting the charge through the fuze _f_ to the earth; _b'_ the cable, attached to another polarised relay _c'_, and connecting the mine with the circuit closer; the polarised relay _c_, in the mine, is arranged to be worked by a positive current, that is to say, the wire surrounding the core is so wound as to increase the polarity of the electro magnet, near the armature _d_, when a positive current is passed through it, and to diminish the polarity when a negative current is passed through the wire surrounding the core; the polarised relay _c'_ within the circuit closer is arranged to be worked by a negative current, the coil being so wound as to produce an influence exactly the reverse of _c_. Then, a positive current passing along the line wire _a_, the armature _d_ in the charge will be attracted, while _d'_ will remain unaffected; again, if a negative current be circulated, the armature _d'_ within the circuit closer will be attracted, while the armature _d_ will remain unaffected. Two insulated wires forked together are wound round each electro magnet, one a thin wire (_g_ and _g'_) having a considerable resistance, about 1000 ohms, being connected direct to the earth plates _e_ and _e'_, and the other a thick wire (_h_ and _h'_) offering a very small resistance, and so arranged that when the armature is attracted, they may be in contact with and complete the circuit through the armature to earth. The thin wire coils are so arranged that a certain number of Leclanché cells (ten or twelve, as may be desired) will make the electro magnets act, while with fewer cells the current would be too weak, and would therefore pass through them to earth without affecting the armature. By means of the three-coil galvanometer, a table of the deflections, obtained by the foregoing system of testing, should be carefully recorded, when the circuit is known to be in good working order, so that any defect in the circuit would be at once indicated on the application of the various tests, by the results so obtained differing from those originally recorded. When a system of submarine mines is placed in position for the purposes of practice and experiment, every trouble should be taken to endeavour to fix the exact position of any defect that may exist, also to ascertain its magnitude, &c., but in time of war, should a defect exist in the system, no time must be lost in such operations, but the mine at once lifted, and the fault repaired, or a fresh one laid in its place, unless the presence of an enemy or other imperative cause should prevent such work being done.