The line of force, in physics, is the path followed by an electric charge free to move in an electric field or generally any appropriate test particle in a given force field.
More abstractly, lines of force are lines in any such force field the tangent of which at any point gives the field direction at that point
and the density of these field lines gives the magnitude of the field.
To investigate the nature of electric force between charges -** Coulomb had performed an experiment using Torsion balance and the inverse square law could be verified for electric field between charges .**
The concept of lines of force was introduced into physics in the 1830s by the English scientist Michael Faraday, who considered magnetic and electric effects in the region around a magnet or electric charge as a property of the region rather than an effect taking place at a distance from a cause.
The electric lines of force that represent the field of a positive electric charge in space consist of a family of straight lines radiating uniformly in all directions from the charge where they originate.
A second positive charge placed in the field would travel radially away from the first charge.
By relating the strength of the Electric field to the density of lines of force crossing unit area of a surface perpendicular to the flux lines led to inverse square dependence on radial distance.
The concept of Maxwell's tubes of forces were also defined and as area crossing through the field lines or tubes varies as the inverse square of the distance(thereby the density of lines of force will decrease as inverse square) it was understood that the nature of field strength will vary as 1/ r^2.