Very qualitatively, flux is a measure of how much of a vector field passes perpendicularly through a given area.  A vector field is simply a vector quantity that has possibly different values (in magnitude and/or direction) at different points in space.  Examples of vector fields are the velocity of air molecules in a room or of water molecules in a stream, the acceleration due to gravity at various locations on and above the earth’s surface, the electric field surrounding a charged balloon, or the magnetic field in the region around a solenoid with current flowing through its windings.  (This explains why the latter two examples are called fields;  we can also speak of the gravitational field around the earth, which is equivalent to the third example listed.) In all of these cases, the vector quantity being described varies with position in space.

The magnetic field through a loop can be changed either by changing the magnitude of the field or by changing the area of the loop. To be able to quantitatively describe these changes, magnetic flux is defined as Φ = B A cosθ, where θ is the angle between B and the direction perpendicular to the plane of the loop (along the axis of the loop).

In 1824, Oersted discovered that current passing though a coil created a magnetic field capable of shifting a compass needle. Seven years later, Faraday and Henry discovered just the opposite. They noticed that a moving magnetic field would induce current in an electrical conductor. This process of generating electrical current in a conductor by placing the conductor in a changing magnetic field is called electromagnetic induction or just induction. It is called induction because the current is said to be induced in the conductor by the magnetic field.

Faraday also noticed that the rate at which the magnetic field changed also had an effect on the amount of current or voltage that was induced. Faraday's Law for an uncoiled conductor states that the amount of induced voltage is proportional to the rate of change of flux lines cutting the conductor.

The voltage (E.M.F.) created in a coil by a changing magnetic field is proportional to rate in which the magnetic flux changes and the number of winding in the coil.