A magnetic needle of pole strength 203  Am is pivoted at its centre. Its North pole is pulled  eastward by a string. The horizontal force required to produce a deflection of  300from magnetic meridian  (take  BH=10−4T)

from the figure   cosθ=yl,y=lcosθ                   Torque due to F is balanced by torque due to BH.⇒Ttension =TBH⇒Fy=MBHsinθ⇒Flcosθ=m(2l)BHsinθ⇒F=2mBHtanθ   Given  m=203Am                BH=10-4Tθ=30°Using, F=2mBHtanθ⇒F=2×203×10-4×13 = 4×10-3 N