Magnetic Circuit

Φ - Magnetic Flux (Weber)
mmf - Magnetomotive Force (Ampere-turns)
N - Number of turns of the coil (turns)
I - Electrical current flowing through the coil (Ampere)
ℜ - Reluctance (1/Henry)

H - Magnetic Field Strength (Ampere-turn/meter)
l - Mean length of flux lines (meter)
mmf - Magnetomotive Force (Ampere-turn)

ℜ - Reluctance (1/Henry)
l - Mean length of the (meter)
μ_o - Permeability of free space = 4π X 10^-7 (N/A²)
μ_r - Relative permeability of the core
A - Cross-sectional area of the magnetic path (m²)
v - Reluctivity