Find the current through resistor R.

Explanation

B due to straight parts of the wire is zero at P. Each arc subtends an angle
`theta=(2pi)/8=pi//4`
`thereforeB_("net")=4((mu_(0)I)/(4pir)xxpi//4)+4((mu_(0)I)/(4pi(2r))xxpi//4)=(3mu_(0)I)/(8r),`
perpendicular to the plane of the paper and directed inward.