Apparatus Competition

2006 AAPT Summer Meeting

Syracuse, NY

A Simple and Inexpensive Apparatus to
Demonstrate The Magnetic Field Produced by a
Current-Carrying Wire Segment

A. Tomasch, D. Gerdes, and M. Love

Department of Physics

Randall Laboratory

The University of Michigan

450 Church Street

Ann Arbor, MI 48109-1040

734-936-2959

atomasch ‘at’ umich ‘dot’ edu

Abstract:

We describe a simple apparatus to quantitatively demonstrate the magnetic field produced by a current-carrying wire segment. The apparatus is very simple and inexpensive to construct and relies on a six volt lantern battery connected to a normally open push button to produce currents of ~10 amperes for brief periods. Students can verify the circular nature of the magnetic field lines surrounding the wire and the right hand rule for the magnetic circulation given the direction of the current. The field strength can also be mapped as a function of distance from the wire to test the 1/r radial dependence of the magnetic field magnitude predicted by Ampere's law. The data typically yield a radial dependence somewhat steeper than r^-1 due to the finite length of the wire.

Description

This is an exceptionally simple apparatus to construct. A small table is constructed from a commercial plastic cutting board and 1” Lucite rod support legs. An aluminum welding rod wire segment is mounted with banana binding posts and supported by Lucite posts and strips, passing through the table top. Current is supplied to banana binding posts by a 6 volt lantern battery connected with banana/alligator clip leads. A normally open pushbutton, mounted on the top support strip, provides current from the battery on demand. Small compasses placed around the wire demonstrate the circular geometry of the magnetic field lines surrounding the wire segment.

If a Hall effect probe is available, the strength of the magnetic field as a function of the radial distance from the wire segment can be mapped. A picture of the apparatus is shown below.