Using the Thomson Jumping Ring to Study the Effect of Eddy Currents and Skin Depth on Ring Jump Height

Since its debut in Elihu Thomson’s 1886 article “Novel Phenomena of Alternating Currents,” the Thomson jumping ring apparatus has been a popular and captivating demonstration of magnetic induction. The components are quite simple. There is a solenoid, an iron core, and a ring. The demonstration usually begins with the professor saying something along the lines of, “An alternating current introduced to the solenoid will create a changing magnetic field, which induces a current in the ring in such a way to oppose the changing flux of the magnetic field responsible for the creation of the ring current. This will make the ring jump!” The professor then turns a switch, the ring is instantaneously launched into the air, and a classroom of students is amazed. The behavior of the demonstration is then attributed Lenz’s law and the lecture moves on.