When this coil is wound in layers, like the thread upon a spool, it increases the intensity of the magnetism in the core by as many times as there are coils, up to a certain point. If the core is merely soft iron, and not steel, it becomes magnetized instantly, as stated, and will draw another piece of iron to it with a snap, and hold it there as long as there is a current passing through the coil. But as instantly, when the current is stopped, this soft iron core ceases to be a magnet, and becomes as it was before--an inert and ordinary piece of iron. What has just been described is always, in some form, one of the indispensable parts of the electromagnetic machines used in industrial electricity, and in all of them except the appliances of electric lighting, and even in that case it is indispensable in producing the current which consumes the points of the carbon, or heats the filament to a white glow. The current may traverse the wire for a hundred miles to reach this little coil. But, instantly, at a touch a hundred miles away that forms a contact, there is a continuous "circuit;" the core becomes a magnet, and the piece of iron near it is drawn suddenly to it. Remove the distant finger from the button, the contact is broken, and the piece of iron immediately falls away again. It is the wonder of the production of instant movement at any distance, without any movement of any connecting part. It is a mysterious and incredible transmission of force not included among human possibilities forty years ago. It is now common, old, familiar. Conceive of its possibilities, of its annihilation of time and space, of its distant control, and of that which it is made to mean and represent in the spelled-out words of language, and it still remains one of the wonders of the world: the Electric Telegraph.
MAGNETS AND MAGNETISM.--Having described a magnet that is made and unmade at will, it may be appropriate to describe magnets generally. The ordinary, permanent magnet, natural or artificial, has little place in the arts. It cannot be controlled. In common phrase, it cannot be made to "let go" at will. The greatest value of magnetism, as connected with electricity, consists in the fact of the intimate relationship of the two. A magnet may be made at will with the electric current, as described above. A little later we shall see how the process may be reversed, and the magnet be made to produce the most powerful current known, and yet owe its magnetism to the same current.
The word Magnet comes from the country of Magnesia, where "loadstone" (magnetic iron ore) seems first to have been found. The artificial magnet, as made and used in early experiments and still common as a toy or as a piece in some electrical appliances, is a piece of fine steel, of hard temper, which has been magnetized, usually by having had a current passed through or around it, and sometimes by contact with another magnet. For the singular property of a magnet is that it may continually impart its quality, yet never lose any of its own. Steel alone, of all the metals, has the decided quality of retaining its property of being a magnet. A "bar" magnet is a straight piece of steel magnetized. A "horseshoe" magnet is a bar magnet bent into the form of the letter "U."
Every magnet has two "poles"--the positive, or North pole, and the negative, or South pole. If any magnet, of any size, and having as one piece two poles only, be cut into two, or a hundred pieces, each separate piece will be like the original magnet and have its two poles. The law is arbitrary and invariable under all circumstances, and is a law of nature, as unexplainable and as invariable as any in that mysterious code. All bar magnets, when suspended by their centers, turn their ends to the North and South, a familiar example of this being the ordinary compass. But in magnetism, like repels like. The world is a huge magnet. The pole of the magnet which points to the North is not the North pole of the needle as we regard it, but the opposite, the South.
No one can explain precisely why iron, the purer and softer the better, becomes a powerful and effective magnet under the influence of the current, and instantly loses that character when the current ceases, and why steel, the purer and harder the better, at first rejects the influence, and comes slowly under it, but afterwards retains it permanently. Iron and steel are the magnetic metals, but there is a considerable list of metals not magnetic that are better than they as conductors of the electric current. In a certain sense they are also the electric metals. A Dynamo, or Motor, made of brass or copper entirely would be impossible. All the phenomena of combined magnetism and electricity, all that goes to make up the field of industrial electric action, would be impossible without the indispensable of ordinary iron, and for the sole reason that it possesses the peculiar qualities, the affinities, described.
There is now an understanding of the electro-magnet, with some idea of the part it may be made to play in the movement of pieces, parts, and machines in which it is an essential. It has been explained how soft iron becomes a magnet, not necessarily by any actual contact with any other magnet, or by touching or rubbing, but by being placed in an electric field. It acquired its magnetism by induction; by drawing in (since that is the meaning of the term) the electricity that was around it. But induction has a still wider field, and other characteristics than this alone. Some distinct idea of these may be obtained by supposing a simple case, in which I shall ask the reader to follow me.
Let us imagine a wire to be stretched horizontally for a little space, and its two ends to be attached to the two poles of an ordinary battery so that a current may pass through it. Another wire is stretched beside the first, not touching it, and not connected with any source of electricity. Now, if a current is passed through the first wire a current will also show in the second wire, passing in an opposite direction from the first wire's current. But this current in the second wire does not continue. It is a momentary impulse, existing only at the moment of the first passing of the current through the wire attached to the poles of the battery. After this first instantaneous throb there is nothing more. But now cut off the current in the first wire, and the second wire will show another impulse, this time in the same direction with the current in the first wire. Then it is all over again, and there is nothing more. The first of these wires and currents, the one attached to the battery poles, is called the Primary. The second unattached wire, with its impulses, is called the Secondary.