Let a ring of radius a carry a current I, and set up cylindrical coordinates
. Let
be the angle between the z axis and the observer.
On-axis, in cgs
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where c is the speed of light, so
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In MKS,

where
is the permeability of free space, so

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Off-axis in MKS, the magnetic vector potential is

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

where

and K(k) and E(k) are complete elliptic integrals of the first and second kind with modulus k.


On-axis, in cgs

(1)
where c is the speed of light, so



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
(2)
In MKS,

(3)
where



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

(4)
Off-axis in MKS, the magnetic vector potential is

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
(5)
where

(6)
and K(k) and E(k) are complete elliptic integrals of the first and second kind with modulus k.
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