FLUX DENSITY INSIDE A LONG COIL

Suppose that you have a long coil of wire, commonly known as a solenoid, with n turns and whose length in meters is s. Suppose that this coil carries a direct current of I amperes and has a core whose permeability is μ. The flux density B in teslas inside the core, assuming that it is not in a state of saturation, can be found using this formula:
B = 4π × 10^-7 (μnI/s)
A good approximation is
B = 1.2566 × 10^-6 (μnI/s)