Abstract: The formula for the production of magnetic field in space by rectangular carrier coil is derived according to Safaer's law. The formula is used to calculate the distribution of magnetic field when two and four rectangular flow line rings have opposite magnetic poles of the same name. According to the calculation results，a magnetic field energy conversion demo instrument is designed to analyze the movement characteristics of the projectile in the magnetic field and the source of kinetic energy.

Key words: Biot-Savart's law, flow coil, spatial magnetic field distribution, energy conversion