Number of horizontal wires in the telephone cable, n=4 Current in each wire, I=1.0 A Earth's magnetic field at a location , H=0.39 G=0.39 x {10^{ - 4}} T Angle of dip at the location, \\delta = {35^ \\circ } Angle of declination, \ heta \\sim {0^ \\circ } For a point 4 cm below the cable:Distance, r=4 cm=0.04 m The horizontal component of earth's magnetic field can be written as: {H_h} = H\\cos \\delta - B Where ,B=Magnetic field at 4 cm due to current I in the four wires = 4 \ imes \\frac{{{\\mu _0}I}}{{2\\pi r}} {\\mu _0} = Permeability of the free space = 4\\pi \ imes {10^{ - 7}}Tm\\,{A^{ - 1}} \ herefore B = 4 \ imes \\frac{{4\\pi \ imes {{10}^{ - 7}} \ imes 1}}{{2\\pi \ imes 0.04}} B = 2 \ imes {10^{ - 9}}