First, in the case of a dry charge with the insulation and conductivity of the cable, good; under these circumstances there would be formed a sea cell between the earth plates x, and c, which would produce a certain deflection of the needle of a galvanometer g, which is placed in the circuit, and in a certain direction.
Secondly, in the case of a charge becoming wet, through leakage, with the insulation and conductivity of the cable, good; under these circumstances, a sea cell would be formed between the plates c and z, causing a different deflection of the needle in amount and in direction, by which it would be at once indicated that the charge had become wet.
TEST TABLE, DIFFERENTIAL GALVANOMETER.
"Sea cell" Test for Insulation.—Again, in the case of the insulation of the electric cable being damaged to such an extent as to expose the copper conductor. Under these circumstances there would be formed a sea cell between the copper earth plate c, and the exposed copper conductor of the cable, by which a certain definite deflection of the galvanometer would be observed, which deflection would differ in character from that produced by the copper carbon sea cell, when the insulation of the cable was good, and the system in working order, and therefore it would indicate that some change in the electrical conditions of the system had occurred. The fact that a leak existed in the insulation would be proved by changing the earth plate at home from copper to zinc, carbon, tin, &c.
In the case of no deflection being produced on the galvanometer, on applying the sea cell test, a want of continuity, or inefficient connections would be indicated.
The foregoing afford examples of the vast utility of the "sea cell" in connection with a system of electrical tests for submarine mines, numerous variations of which may be effected by employing a series of earth plates, of different metals, at the home end of the circuit, in connection with a carbon and zinc earth plate at the other end. And the mode of manipulating these tests may, by means of numerous switch plates, as shown at [Fig. 95], be made extremely simple and efficient.
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.