Lectures on Physics - Volume II - Richard Feynman
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Equation (1.9) tells us that for a fixed current through the wire the circulation of B is the same for any curve that surrounds the wire. For curves - say circles - that are farther away from the wire, the circumference is larger, so the tangential component of B must decrease. You can see that we would, in fact, expect B to decrease linearly with the distance from a long straight wire.
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