McEvoy's Single Main System.—Hitherto in connection with a system of electrical submarine mines, it has been necessary to employ either a single cable between each submarine mine and the torpedo station, or a single cable, termed a "multiple cable," containing a limited number of insulated wires, leading from the station, and branching off from a junction box to each mine, by which considerable cost and complication is incurred. To remedy the above serious defects of such a system, and also to simplify the arrangement of electrical tests, Captain McEvoy has devised and patented the following apparatus; at the firing, or torpedo station, the end of the single main cable, that is, the single core cable leading to the junction box, is connected to a make and break contact apparatus, by which, by the movement of a dial or pointer around a fixed centre, a battery can be successively put in connection with the wire, and disconnected from it, in a somewhat similar manner to Wheatstone's step by step dial telegraphs. In the junction box at the opposite end of the single core main cable is an electro-magnetic apparatus for working a dial or pointer in exact unison with the aforesaid dial or pointer at the torpedo station. This junction box dial or pointer serves as a contact maker to put the wire of the main cable successively in contact with the branch wires leading to the several torpedoes, as it is caused to turn with a step by step motion by the sending of a succession of currents from the firing station.

As the contact maker completes the circuit between the main cable and one of the branch wires, the current passes from the cable through the wire, and through the fuze of that particular torpedo to "earth"; but when any one or other of the torpedoes is to be exploded, the circuit between the main cable and the torpedo wire being completed, it is only necessary to send a current through the main cable of sufficient strength to ignite the fuze, and so explode the mine.

The strength of the current used for giving the aforesaid step by step motion to the junction box dial or pointer is not sufficient to cause the ignition of the fuzes in the torpedoes.

Again, if it be desired that the torpedoes should be so arranged that when any of them are struck by a passing vessel, the fact of its having been struck should be instantly signalled to the firing station. The dial apparatus in the junction box is arranged so that at one point of its revolution, termed the "zero point," all the torpedo branch wires are in circuit with the main cable, and that then a constant current is passing from the firing station through all the circuit closers, and out through resistance coils to "earth." In this case, if one of the circuit closers be struck, and therefore short circuit formed, the current passes direct to earth without going through the aforesaid resistance, and the fact of its having done so is at once indicated by a galvanometer at the firing point, by the movement of which a bell is rung at the station. The operator can then explode such torpedo at once by merely switching in the firing battery.

At the same time the passage of the strong firing current may fuze a connection in the junction apparatus, by which the exploded torpedo is detached, i.e. the direct "earth" connection of such a torpedo is cut off, and the remaining submarine mines are left in proper working order; this effect may also be arrived at by other means.

General Description of Apparatus.—The following is a general description of this exceedingly clever and useful invention:—

At Fig. 168 is shown a diagram view of the apparatus.

A is the instrument at the firing point on the shore or vessel; B is the cable wire led to a submerged box situated near the spot where the several torpedoes are grouped; C is the instrument enclosed in the submerged box; D, D are insulated wires led away from the box to the several torpedoes, there being a separate wire for each torpedo.

Each of the wires D is coupled to one or other of a series of metallic contact pieces E ranged in a circle round the axis of a metallic pointer F, which can be turned with a step by step motion and successively brought into electrical contact with the several contact pieces E. The axis of the pointer is in electrical communication with the wire of the cable. The wire from the cable is first led to the coils of an electro magnet G, and thence passes to the axis of the pointer. H is a magnetic armature in front of the electro magnet G; when a positive current of sufficient strength is sent through the cable the armature is rocked in one direction, and when a negative current is sent, it is rocked in the opposite direction. From the armature motion is transmitted to a pawl which works into the teeth of a ratchet wheel on the axis of the pointer F, so that by sending a succession of reversed currents of sufficient strength through the cable, the pointer F is turned with a step by step motion and is successively brought into electrical contact with the several contact pieces E.

In the instrument, at the firing point a is a handle, by the turning of which a step by step motion is given to the pointer of a dial b and a simultaneous movement to the pointer F of the instrument C in the submerged box. When the handle a has made a half turn it couples one pole of the battery to the cable and the other to the earth connection, and when it has made a complete turn the connections are reversed. The pointer of the dial b then moves forward from one division of the dial to the next, and simultaneously the pointer F is turned in unison with it. The operator at the firing point can therefore always see which of the torpedoes is in electrical connection with the wire of the cable, and he can test each torpedo in succession by moving a handle, say at h, to cause the current passing back from the torpedo to pass through a galvanometer at e, and by the movement of the needle of the galvanometer it can be seen whether the resistance of the circuit through this torpedo is in its normal and proper working state.

When the pointer of the dial b is brought to zero, or as it is marked in the drawing to "signal," then the pointer F of the apparatus C is in electrical communication with a contact point which is coupled to all of the branch wires D, and usually the apparatus is left in this condition, the handle a being then locked and prevented from turning by a bolt actuated by a handle at G.

The current from the battery at the firing point then passes to earth through the resistances in all of the torpedoes. If now any one or other of the torpedoes is struck by a passing vessel and the wire from its fuze put directly to earth, so that the current passes freely to earth instead of having first to pass through the resistance, the fact of the current passing freely to earth is notified at the firing point by the movement of the needle of a galvanometer d; the movement of the needle of this galvanometer effects an electrical connection by which a small battery is caused to sound a bell at c. The operator at the firing point can then if he pleases at once fire the torpedo that has been struck by moving a handle at f and coupling up to the wire of the cable a battery of greater strength; the strong firing current will pass to earth through the fuze of the torpedo that has been struck, and will ignite this fuze, but will not affect the fuzes of the other torpedoes, as to pass through these fuzes it has also to pass through resistances which impede its passage and reduce its strength, so that the portion of the current which passes to earth through them is not of sufficient strength to ignite the fuzes.

When the fuze of any one or other of the torpedoes is exploded by the passing of a strong firing current through it, the wire leading from the box C to this torpedo is simultaneously cut off from electrical connection with the contact pin E to which it was previously connected, and this pin is put to earth through a resistance either somewhat greater or less than the resistances in the torpedoes, so that the firing of one or more of the torpedoes does not interfere with the power of being able to turn the pointer F of the apparatus C in unison with the pointer of the dial b.

Afterwards the operator at the firing point can ascertain which of the torpedoes has been fired by passing the pointer of the dial b to each of the divisions of the dial in succession, and ascertaining by the galvanometer a the resistance of the circuit through each of the torpedoes, so that he at once ascertains which torpedo has been put to earth through the greater or less resistance.

The cutting off of the wire D from its contact E when a strong current is passed through it may be effected by the wire being coiled around an iron core forming an electro magnet, which when a strong current is passed through the wire is of sufficient strength to shift the position of a contact apparatus and then effect the required alterations in the connections, but which is not of sufficient strength to effect any change when the weaker currents used for the signalling and testing operations are passed through the wire.

It will be evident that with the above described apparatus any one or other of the torpedoes can if desired be exploded by the operator at the firing point whenever he desires to do so. To effect this he would by turning the handle a bring the pointer of the dial b opposite to the division of this dial; that would indicate that the cable had been brought into electrical communication with the torpedo required to be exploded, and then when it is ascertained by previously adjusted sight points that the vessel is above the torpedo, he can fire the torpedo by passing a strong firing current to the cable.

In this way the apparatus can be used for firing any one or other of a group of sunken torpedoes, or if the torpedoes are buoyant ones, they need not be fitted with apparatus for putting the wire from their fuze directly to earth whenever the torpedo is struck by a passing vessel. The same arrangement of apparatus can also be used for firing any one or other of a number of mines or torpedoes on land and for separately testing the firing mechanism of each mine whenever desired.

Captain McEvoy's single main system will shortly undergo a series of experiments under the supervision of the English torpedo authorities at Chatham, which will most probably result in its adoption by the English government, and also by the principal continental powers.