This chapter covers magnetic phenomena caused by currentsCurrent. The Lorentz forceLorentz force, which works between currentsCurrent, is caused by a magnetic distortion in space that is produced by the currentsCurrent. This distortion is the magnetic fieldMagnetic field and is represented by the magnetic flux densityMagnetic flux density. The magnetic flux densityMagnetic flux density produced by currentCurrent is described by the Biot-Savart lawBiot-Savart law. On the other hand, Ampere’s lawAmpere’s law describes the global relationship between the currentCurrent and the magnetic flux densityMagnetic flux density. The magnetic flux densityMagnetic flux density is expressed by the curlCurl of the vector potentialVector potential and has a nature of a field with no divergenceDivergence. The virtual surface on which the vector potentialVector potential is the same is the equivector potential surface. The relationship between the equivector potential surface and the magnetic flux densityMagnetic flux density is discussed. We also learn the magnetic properties of the magnetic momentMagnetic moment produced by a small closed currentSmall closed current, which appears in magnetic materialsMagnetic material.