B B =L B 2IT R 2TR 2兀r B R r=2r B P=IS P=R Pr=2Tr l P P R 20 0 1 2 , 2 . 2 2 I I B B R r     = = R = 2r 2 4 1 2 = = r R B B Pm = IS , 2 . 2 2 P R I P r I m = m  =  2 1 2 2 2 = =  R r P P m m 0 2 I B R   =