At the present day, an electrical torpedo may be described as consisting of a strong, water-tight vessel of iron or steel, which contains a large amount of some explosive, usually gun-cotton, and a device for detonating this explosive by electricity.  The old mechanical mine used in our civil war did not know a friendly ship from a hostile one, and would sink either with absolute impartiality.  But the electrical submarine mine, being exploded only when a current of electricity is sent through it from ship or shore, makes no such mistake, and becomes harmless when detached from the battery.  The condition of the mine at any time can also be told by sending a very minute current through it, though miles away and buried deep beneath the sea.

When a current of electricity goes through a wire, it heats it; and if the current be made strong enough, and a white hot wire thus comes in contact with powder or fulminate of mercury in a torpedo, an explosion will result.  But it is important to know exactly when to explode the torpedo, especially during the night or in a fog; and hence torpedoes are often made automatic by what is called a circuit closer.  This is a device which automatically bridges over the distance between two points which were separated, thus allowing the current to pass between them.  In submarine torpedoes it is usual to employ a small weight, which, when the torpedo is struck, is thrown by the force of the blow across two contact points, one of which points is in connection with the fuse and the other in connection with the battery, so that the current immediately runs over the bridge thus offered, and through the fuse.  In practice, these two contact points are connected by a wire, even when the torpedo is not in the state of being struck; but the wire is of such great resistance that the current is too weak to heat the wire in the fuse.  Yet when the weight above mentioned is thrown across the two contact points, the current runs across the bridge, instead of through the resistance wire, and is then strong enough to heat the wire in the fuse and explode the torpedo.  The advantage of having a wire of high resistance between the contact points, instead of having no wire between them, is that the current which then passes through the fuse, though too weak to fire it, shows by its very existence to the men on shore that the circuit through the torpedo is all right.

But instead of having the increased current caused by striking the torpedo to fire the torpedo directly, a better way is to have it simply make a signal on shore.  Then, when friendly vessels are to pass, the firing battery can be disconnected; and when the friendly ship bumps the torpedo, the working of the signal shows not only that the circuit through the fuse is all right, but also that the circuit closer is all right, so that, had the friendly ship been a hostile ship, she would certainly have been destroyed.

While the management of the torpedo is thus simple, the defense of a harbor becomes a complex problem, on account of the time and expense required to perfect it, and the training of a corps of men to operate the torpedoes.