PURPOSE: To show that a magnetic field is created by a current wire.
DESCRIPTION: A stiff wire is clamped to stand upright on the overhead as shown in the photo at left. A low voltage DC power supply provides a 10 amp current to the wire. A compass needle probed around the wire will show the direction of the magnetic field around the wire due to the current, as shown in the middle photo. The magnetic field can also be visualized by sprinkling iron filings around the wire.
The Ørsted apparatus, shown in the photo at right, can also be used.
While preparing for an evening lecture on 21 April 1820, Ørsted developed an experiment which provided evidence that surprised him. As he was setting up his materials, he noticed a compass needle deflected from magnetic north when the electric current from the battery he was using was switched on and off. This deflection convinced him that magnetic fields radiate from all sides of a wire carrying an electric current, just as light and heat do, and that it confirmed a direct relationship between electricity and magnetism.
At the time of discovery, Ørsted did not suggest any satisfactory explanation of the phenomenon, nor did he try to represent the phenomenon in a mathematical framework. However, three months later he began more intensive investigations. Soon thereafter he published his findings, proving that an electric current produces a magnetic field as it flows through a wire. The CGS unit of magnetic induction (oersted) is named in honor of his contributions to the field of electromagnetism.
His findings resulted in intensive research throughout the scientific community in electrodynamics. The findings influenced French physicist André-Marie Ampère's developments of a single mathematical form to represent the magnetic forces between current-carrying conductors. Ørsted's discovery also represented a major step toward a unified concept of energy.
EQUIPMENT: The large wire loop is stored in the electricity and magnetism cabinet.
SETUP TIME: Place the compass needle to point at the wire when the current is off and only the earth's field is present. This way when the power switched is turned on the needle will deflect to point perpendicular to the wire. Also, keep the power supply at some distance from the needle or the transformer may influence the magnetic field. Also, some overheads contain permanently magnetized transformer laminations that will influence the direction of the compass needle.
Updated by Jun Qi in 3/13/2000