Faraday first represented the electric field around an electrically charged body by means of lines which he called lines of force or lines of electric intensity. These lines are so drawn that at every point in the field the direction of the line tangent to the line of force in this point coincides with the direction of the intensity vector at that point. Thus, a positive unit of electric charge will tend to move along a line of force in the direction of the electric intensity, and a negative unit of charge tends moving in the opposite direction.

The number of lines of force that are radiating from a unit charge may be found by placing the unit charge at the centre of a sphere of one centimeter radius placed in a uniform isotropic dielectric. By the way, do you know what this term means? An isotropic medium is one whose properties are the same in all directions.

It is obvious that two lines of force can never meet, for then we should have the intensity at the point of meeting acting in two directions at once. In addition to that, we know of their being continuous at long as there a dielectric medium exists.

At any rate, one should always remember that lines of force do not really exist and that they by no means indicate the structure of the medium. We use these lines of force, as did Faraday, in order to picture the electric field more clearly. Representing the electric field by means of these lines helps us in picturing the forces existing between the charged bodies.

Magnetic Lines of Force. Magnetism manifests itself as if it existed in lines emanating from the magnetic materials or current-carrying conductors, these lines being called magnetic lines of force. The stronger the magnet, the more of magnetic lines of force passes through a given space. The magnetic lines taken as a whole constitute a magnetic flux.

Just like the lines of force in an electric circuit, magnetic lines of force do not exist in reality. One can imagine magnetic lines as emitted from the north pole and passing through the air to the south pole. However, they do not end on magnetic poles but continue passing from the south to the north pole. The magnetic lines of force are more than merely a means of showing the presence and direction of the magnetic field. They may serve as a unit of measurement since the magnetic field strength is measured by the number of lines of force running through a square centimeter.

Fig. shows how the lines of force are distributed between the like poles and between the unlike poles of two magnets.