Introduction Magnetic fields, B, can be produced by moving charges (such as currents) and in turn produce forces on moving charges. In this lab you will measure the magnetic field produced at the center of a coil of wire carrying a current and compare it to the earth’s magnetic field. Equipment Dip needle, compass, tangent galvanometer (coil), DC power supply, cables, ammeter (or use internal readout on Hewlett-Packard supplies). Note: steel lab tables and chairs will interfere with the Earth’s field. If the lab benches are steel, this experiment must be conducted on the floor away from steel tables and chairs. Theory A magnetic moment is the source of the magnetic field. The simplest type of magnetic moment is the magnetic dipole: magnetic field lines go from the North pole to the South pole. The Earth has a permanent magnetic moment. The geographic North Pole of the earth is actually its magnetic “south” pole since the north pole of a compass needle will point North. Atoms can have small magnetic moments which produce small magnetic fields. These are due to both the intrinsic magnetism of the protons, neutrons and electrons and to the circulation of the electrons about the nucleus. In most cases the magnetic moments of the atoms are randomly aligned so that on average there is no net magnetic field for the material.