Submarine mines may be divided into two classes; those for harbour defence, and those for use in the open sea. Harbour defence mines are almost invariably electrically controlled; that is, they are connected with the shore by means of a cable, and fired by an electric impulse sent along that cable. In one system of control the moment of firing is determined entirely by observers on shore, who, aided by special optical instruments, are able to tell exactly when a vessel is above any particular mine. The actual firing is carried out by depressing a key which completes an electric circuit, thus sending a current along the cable to actuate the exploding mechanism inside the mine. A hostile ship therefore would be blown up on arriving at the critical position, while a friendly vessel would be allowed to pass on in safety. In this system of control there is no contact between the vessel and the mine, the latter being well submerged or resting on the sea floor, so that the harbour is not obstructed in any way. This is a great advantage, but against it must be set possible failure of the defence at a critical moment owing to thick weather, which of course interferes seriously with the careful observation of the mine field necessary for accurate timing of the explosions. This difficulty may be surmounted by a contact system of firing. In this case the mines are placed so near the surface as to make contact with vessels passing over them. The observers on shore are informed of the contact by means of an electric impulse automatically transmitted along the cable, so that they are independent of continuous visual observation of the mined area. As in the previous system, the observers give the actual firing impulse. The drawback to this method is the necessity for special pilotage arrangements for friendly ships in order to avoid unnecessary striking of the mines, which are liable to have their mechanism deranged by constant blows. If the harbour or channel can be closed entirely to friendly shipping, the observers may be dispensed with, their place being taken by automatic electric apparatus which fires at once any mine struck by a vessel.
Shore-controlled mines are excellent for harbour defence, and a carefully distributed mine-field, backed by heavy fort guns, presents to hostile vessels a barrier which may be regarded as almost impenetrable. A strong fleet might conceivably force its way through, but in so doing it would sustain tremendous losses; and as these losses would be quite out of proportion to any probable gains, such an attempt is not likely to be made except as a last resort.
For use in the open sea a different type of mine is required. This must be quite self-contained and automatic in action, exploding when struck by a passing vessel. The exploding mechanism may take different forms. The blow given by a ship may be made to withdraw a pin, thus releasing a sort of plunger, which, actuated by a powerful spring, detonates the charge. A similar result is obtained by the use of a suspended weight, in place of plunger and spring. Still another form of mine is fired electrically by means of a battery, the circuit of which is closed automatically by the percussion. Deep-sea mines may be anchored or floating free. Free mines are particularly dangerous on account of the impossibility of knowing where they may be at any given moment. They are liable to drift for considerable distances, and to pass into neutral seas; and to safeguard neutral shipping international rules require them to have some sort of clockwork mechanism which renders them harmless after a period of one hour. It is quite certain that some, at least, of the German free mines have no such mechanism, so that neutral shipping is greatly endangered.
Submarine mines are known as ground mines, or buoyant mines, according to whether they rest on the sea bottom or float below the surface. Ground mines are generally made in the form of a cylinder, buoyant mines being usually spherical. The cases are made of steel, and buoyancy is given when required by enclosing air spaces. Open-sea mines are laid by special vessels, mostly old cruisers. The stern of these ships is partly cut away, and the mines are run along rails to the stern, and so overboard. The explosive employed is generally gun-cotton, fired by a detonator, charges up to 500 lb. or more being used, according to the depth of submersion and the horizontal distance at which the mine is desired to be effective. Ground mines can be used only in shallow water, and even then they require a heavier charge than mines floating near the surface. Mines must not be laid too close together, as the explosion of one might damage others. The distance apart at which they are placed depends upon the amount of charge, 500-lb. mines requiring to be about 300 feet apart for safety.
CHAPTER XXXI
WHAT IS ELECTRICITY?
The question which heads this, our final chapter, is one which must occur to every one who takes even the most casual interest in matters scientific, and it would be very satisfactory if we could bring this volume to a conclusion by providing a full and complete answer. Unfortunately this is impossible. In years to come the tireless labours of scientific investigators may lead to a solution of the problem; but, as Professor Fleming puts it: “The question—What is electricity?—no more admits of a complete and final answer to-day than does the question—What is life?”
From the earliest days of electrical science theories of electricity have been put forward. The gradual extension and development of these theories, and the constant substitution of one idea for another as experimental data increased, provide a fascinating subject for study. To cover this ground however, even in outline, would necessitate many chapters, and so it will be better to consider only the theory which, with certain reservations in some cases, is held by the scientific world of to-day. This is known as the electron theory of electricity.
We have referred already, in [Chapter XXIV]., to atoms and electrons. All matter is believed to be constituted of minute particles called “atoms.” These atoms are so extremely small that they are quite invisible, being far beyond the range of the most powerful microscope; and their diameter has been estimated at somewhere about one millionth of a millimetre. Up to a few years ago the atom was believed to be quite indivisible, but it has been proved beyond doubt that this is not the case. An atom may be said to consist of two parts, one much larger than the other. The smaller part is negatively electrified, and is the same in all atoms; while the larger part is positively electrified, and varies according to the nature of the atom. The small negatively electrified portion of the atom consists of particles called “electrons,” and these electrons are believed to be indivisible units or atoms of negative electricity. To quote Professor Fleming: “An atom of matter in its neutral condition has been assumed to consist of an outer shell or envelope of negative electrons associated with some core or matrix which has an opposite electrical quality, such that if an electron is withdrawn from the atom the latter is left positively electrified.”
The electrons in an atom are not fixed, but move with great velocity, in definite orbits. They repel one another, and are constantly endeavouring to fly away from the atom, but they are held in by the attraction of the positive core. So long as nothing occurs to upset the constitution of the atom, a state of equilibrium is maintained and the atom is electrically neutral; but immediately the atom is broken up by the action of an external force of some kind, one or more electrons break their bonds and fly away to join some other atom. An atom which has lost some of its electrons is no longer neutral, but is electro-positive; and similarly, an atom which has gained additional electrons is electro-negative. Electrons, or atoms of negative electricity, can be isolated from atoms of matter, as in the case of the stream of electrons proceeding from the cathode of a vacuum tube. So far, however, it has been found impossible to isolate corresponding atoms of positive electricity.
From these facts it appears that we must regard a positively charged body as possessing a deficiency of electrons, and a negatively charged body as possessing an excess of electrons. In [Chapter I]. we spoke of the electrification of sealing-wax or glass rods by friction, and we saw that according to the nature of the substance used as the rubber, the rods were either positively or negatively electrified. Apparently, when we rub a glass rod with a piece of silk, the surface atoms of each substance are disturbed, and a certain number of electrons leave the glass atoms, and join the silk atoms. The surface atoms of the glass, previously neutral, are now electro-positive through the loss of electrons; and the surface atoms of the silk, also previously neutral, are now electro-negative through the additional electrons received from the glass atoms. As the result we find the glass to be positively, and silk to be negatively electrified. On the other hand, if we rub the glass with fur, a similar atomic disturbance and consequent migration of electrons takes place, but this time the glass receives electrons instead of parting with them. In this case the glass becomes negatively, and the fur positively electrified. The question now arises, why is the movement of the electrons away from the glass in the first instance, and toward it in the second? To understand this we may make use of a simple analogy. If we place in contact two bodies, one hot and the other cold, the hot body gives up some of its heat to the cold body; but if we place in contact with the hot body another body which is still hotter, then the hot body receives heat instead of parting with it. In a somewhat similar manner an atom is able to give some of its electrons to another atom which, in comparison with it, is deficient in electrons; and at the same time it is able to receive electrons from another atom which, compared with it, has an excess of electrons. Thus we may assume that the glass atoms have an excess of electrons as compared with silk atoms, and a deficiency in electrons as compared with fur atoms.