Equation $(66)$ is obviously also applicable to the exterior of any

conductor of circular cross section carrying a current I in the az direction in

free space. The zero reference is arbitrarily set at $=$ b$.$ Now consider two

conductors, each of $1$ cm radius, parallel to the z axis with their axes lying

in the x $=0$ plane. One conductor whose axis is at $(0,4$ cm$,$ z$)$ carries $12$ A

in the az direction; the other axis is at $(0,4$ cm$,$ z$)$ and carries $12$ A in the

$$az direction. Each current has its zero reference for A located $4$ cm from

its axis. Find the total A field at: $($a$)(0,0,$ z$)$; $($b$)(0,8$ cm$,$ z$)$; $($c$)(4$ cm$,$

$4$ cm$,$ z$)$; $($d$)(2$ cm$,4$ cm$,$ z$).$