The Tolman-Stewart Experiment – A Modern Interpretation

The Tolman-Stewart Experiment, published in 1916 by Richard C. Tolman and T. Dale Stewart, proved that the charge carriers in a conductor were identical to the electrons previously discovered in cathode rays and beta rays. The concept of free electrons in a conductor is still an essential building block of electrodynamics, and the Tolman-Stewart experiment demonstrates this in a fundamental way. Our interpretation of this experiment makes use of modern circuitry and computer control to better adapt the experiment for an advanced undergraduate lab course. We spin a coil of copper or aluminum wire up to sufficiently high speeds and brake it suddenly. This stops the nuclei, but the free electrons keep moving, creating a short-lived current pulse. This current pulse is amplified, converted to a voltage, and recorded using a simple circuit and data acquisition system. Modern rotational encoders accurately determine the rotational speed. Our results confirm that the charge carriers inside both copper (negative charge carriers via the Hall Effect) and aluminum (positive charge carriers via the Hall Effect) are electrons. This interpretation brings a canonical experiment to the advanced lab for the first time and yields compelling proof of the identity of charge carriers in a conductor.