Flux Redux: The Spinning Coil Comes Around Again

Abstract

An essential laboratory exercise for our lower-division electromagnetism course involves the measurement of Earth’s local magnetic field from the emf induced in a rotating coil of wire. Although many methods exist for the measurement of Earth’s field, this one gives our students some practical experience with Faraday’s law. The apparatus we had been using previously for this activity, as has been described in TPT, is a coil driven by an electric motor at constant angular speed with its axis aligned vertically or horizontally. The emf produced was then used to calculate values for the components of the B field. Although this has produced acceptable results, the rotators have proven to be noisy, both electrically and acoustically, as well as somewhat hazardous. Additionally, the effect of stray fields from magnetic materials in the base and housing of the rotator on our measurements has not been quantitatively assessed. Now worn badly by decades of use, these units are prohibitively expensive to replace. In response to this situation, we have designed, built, and tested a quieter, greener, and more compact apparatus that can be produced cost effectively. As part of the experimental design, we have removed the constraint of constant angular rotation rate for the coil; this turns out to provide a much richer data analysis experience for students.