The magnetic field B at the center of a circular coil of wire carrying a current I (as in Fig. 20-9) is

B = μ0 NI / 2r,
where N is the number of loops in the coil and r is its radius. Imagine a simple model in which the Earth's magnetic field of about 1G (= 1 × 10-4 T) near the poles is produced by a single current loop around the equator. Roughly estimate the current this loop would carry.