# At the equator, the earth’s field is essentially horizontal; near the north pole, it is nearly…

At the equator, the earth’s field is essentially horizontal;
near the north pole, it is nearly vertical. In between, the angle
varies. As you move farther north, the dip angle, the angle of the
earth’s field below horizontal, steadily increases. Green turtles
seem to use this dip angle to determine their latitude. Suppose you
are a researcher wanting to test this idea. You have gathered green
turtle hatchlings from a beach where the magnetic field strength is
50 μT and the dip angle is 56∘. You then put the turtles in a 1.4 m
diameter circular tank and monitor the direction in which they swim
as you vary the magnetic field in the tank. You change the field by
passing a current through a 200-turn horizontal coil wrapped around
the tank. This creates a field that adds to that of the earth. What
current should you pass through the coil, to produce a net field in
the center of the tank that has a dip angle of 62 degrees?