Here is one way to measure the strength of the Earth's magnetic field using a compass and a very long wire. When the current in the wire is off, the compass points north. Align the wire so that it runs north-south parallel to the compass needle and arrange it so that current I will travel northward. Hold the compass a height h above the wire and turn on the current. The needle of the compass will twist (in which direction does it go, east/clockwise or west/counterclockwise?); measure the angle ? from the north. What is the strength of the Earth's magnetic field in terms of the above quantities?

M'S