Henry's design was found an error

This interesting study has acquired attention from Nature Physics. The authors reveal an error with Henry's design on display in Princeton University.

In 1831, Henry invented a battery-powered rocking-beam motor that he later described as the first electromagnetic machine. He repeatedly modified the design over his career, but only one version of a motor actually constructed by Henry is known to exist. This version is in a collection of Henry instruments at Princeton University. We found that the Princeton motor cannot have operated in the
form that was displayed as early as 1884. We found evidence in several historical documents and in the instrument itself that the field magnet shown with the motor is a mistake. Instead of a single horizontal bar magnet, the motor was designed to use two elliptical magnets. We presume the error was made by whoever assembled the first public display. We modeled the dynamics of Henry’s vibrating motor and compared our results to the operation of a replica motor. Modeling provides
insight into how the motor is able to vibrate indefinitely even in the presence of energy loss due to friction. © 2011 American Association of Physics Teachers.
DOI: 10.1119/1.3531940