A Rowland ring of mean radius 15 cm has 3500 turns of wire wound on a ferromagnetic core of relative permeability 800. What is the magnetic field B in the core for a magnetising current of 1.2 A?

Mean radius of a Rowland ring, r = 15 cm = 0.15 m

Number of turns on a ferromagnetic core, N = 3500

Relative permeability of the core material, μ =800

Magnetising current, I = 1.2 A

The magnetic field is given by the relation:

B = μ_rμ0 IN/2πr

Where, [μ0 = Permeability of free space = 4π x 10^-7 T m A^-1

B = (800 x 4π x 10^-7 x 1.2 x 3500) / (2πx 0.15) =4.48 T

Therefore, the magnetic field in the core is 4.48 T.