Faraday knew that the space or region around a magnet is permeated or traversed by what he called magnetic curves, or lines of magnetic force.  These lines are still called “lines of magnetic force,” or by some “magnetic streamings” “magnetic flux,” or simply “magnetism.”  They are invisible, though their presence is readily manifested by means of iron filings.  They are present in every magnet, and although we do not know in what direction they move, yet in order to speak definitely about them, it is agreed to assume that they pass out of every magnet at its north-seeking pole (or the pole which would point to the magnetic north, were the magnet free to move as a needle), and, after having traversed the space surrounding the magnet, reenter at its south-seeking pole, thus completing what is called the magnetic circuit.  Any space traversed by lines of magnetic force is called a magnetic field.

But it is not only a magnet that is thus surrounded by lines of magnetic force, or by ether streamings.  The same is true of any conductor through which an electric current is flowing, and their presence may be shown by means of iron filings.  If an active conductor—­a conductor conveying an electric current, as, for example, a copper wire—­be passed vertically through a piece of card-board, or a glass plate, iron filings dusted on the card or plate will arrange themselves in concentric circles around the axis of the wire.  It requires an expenditure of energy both to set up and to maintain these lines of force.  It is the interaction of their lines of force that causes the attractions and repulsions in active movable conductors.  These lines of magnetic force act on magnetic needles like other lines of magnetic force and tend to set movable magnetic needles at right angles to the conducting wire.

The setting up of an electric current in a conducting wire is, therefore, equivalent to the setting up of concentric magnetic whirls around the axis of the wire, and anything that can do this will produce an electric current.  For example, if an inactive conducting wire is moved through a magnetic field; it will have concentric circular whirls set up around it; or, in other words, it will have a current generated in it as a result of such motion.  But to set up these whirls it is not enough that the conducting wire be moved along the lines of force in the field.  In such a case no whirls are produced around the conductor.  The conductor must be moved so as to cut or pass through the lines of magnetic force.  Just what the mechanism is by means of which the cutting of the lines of force by the conductor produces the circular magnetic whirls around it, no man knows any more than he knows just what electricity is; but this much we do know,—­that to produce the circular whirls or currents in a previously inactive conductor, the lines of force of some already existing magnetic field must be caused to pass through the conductor, and that the strength of the current so produced is proportional to the number of lines of magnetic force cut in a given time, say, per second; or, in other words, is directly proportional to the strength of the magnetic field, and to the velocity and length of the moving conductor.