A magnet is suspended in such a way that it oscillates in the horizontal plane. It makes 20 oscillations per minute at a place where dip angle is 30o and 15 oscillations per min. at a place where dip angle is 60o. The ratio of the total earth's magnetic field at the two Places is

Let the total magnetic fields due to earth at the two places be B1 and B2. lf horizontal components be (BH)1 and (BH)2 respectively, thenBH1=B1cos⁡30∘ and BH2=B2cos⁡60∘  Here T1=3 sec. and T2=4 sec.  T1=3=2πIMB1cos⁡30∘ and T2=4=2πIMB2cos⁡60∘ ∴34=B2cos⁡60∘B1cos⁡30∘1/2 or B1B2=169×cos⁡60∘cos⁡30∘ or B1D=160×12×22=160 or B1:B2=16:93