Suppose you are a researcher doing such an experiment in a location where the earth’s field is 50 at a 60 angle below horizontal. You make a 70 diameter, 200-turn coil around a roosting box; the sleeping bats are at the center of the coil. You wish to pass a current through the coil to produce a field that, when combined with the earth’s field, creates a net field with the same strength and dip angle (60 below horizontal) as the earth’s field but with a horizontal component that points south rather than north.