To see why an MRI utilizes iron to increase the magnetic field created by a coil, calculate the current in amperes needed in a 390-loop-per-meter circular coil 0. 660 m in radius to create a 1. 15 T field (typical of an MRI instrument) at its center with no iron present. A The magnetic field of a proton is approximately like that of a circular current 100p0. 650×10⁽⁻¹⁵⁾m in radius carrying 1. 05×10 ⁴ A. What is the field in teslas at the center of such a loop? What current (in A) is needed in a solenoid with 19,000 turns per meter to produce a magnetic fleld 10⁵ times the Earth's magnetic field of 5. 00×10⁽⁻⁵⁾T inside the solenoid?