r m = ( 1 2 ) O D + I D 2 {\displaystyle r_{m}=({\frac {1}{2}}){\frac {OD+ID}{2}}}
H m = ( N i l m ) {\displaystyle H_{m}=({\frac {Ni}{l_{m}}})}
H m = 20 x 2.5 .141 = 354.6 A / m ) {\displaystyle H_{m}={\frac {20x2.5}{.141}}=354.6\ A/m)}
