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A particle moving in a gradually changing magnetic field will develop a drift velocity in a direction perpendicular to both the magnetic field and the direction in which it grows. Let us consider a charged particle going round in a non uniform magnetic field. When it feels a stronger magnetic field its curvature radius would be smaller, and vice versa. Thus, each in cycle the center of each circle will be separated by the difference of radii

Multiplying by the frequency we can obtain the average velocity of the drift

If the gradient in the magnitude of the magnetic field is constant, then

Magnetic drift.png