Definitions for "LINE OF FORCE"
A line in an electric or magnetic field that shows the direction of the force.
The lines that can be produced by a permanent magnet or a use of a sheet of paper and iron filings are called "magnetic lines of force". These lines are imaginary lines, but the concept is convenient for describing a magnetic field. A magnetic field has an intensity of so may lines per square inch. These lines have a definite direction and always form closed loops and return back onto themselves.
an imaginary line in a field of force; direction of the line at any point is the direction of the force at that point