A circular coil with a radius of 0.49 meters is wrapped 846 times and is spun in a magnetic field of 0.11 milli-tesla. It is then connected to the primary side of a transformer with 60 windings on the primary side. The secondary side has 19 windings. If the resistance of the secondary side of the transformer is 8 Ohms. At what frequency, in Hertz, does the coil need to be spun in the field if the maximum current on the secondary side of the transformer is 9 amps?