The Jumping Ring

The Jumping Ring is a classic demonstration of electromagnetic induction.

I usually perform this demonstration at the start of the A level (or IB) Electromagnetic Induction topics (it's a little too advanced for most GCSE pupils) and challenge my pupils to figure it out. I then repeat the demo later on in the topic, once they have covered Faraday's law, and get them to explain it to me.

How does it work?

The coil at the bottom creates a rapidly changing magnetic field in the vertical iron rod (the stand). This field passes through the ring and induces an emf in it, creating a current. Sometimes it helps students to think of what would happen if you replaced the ring with a second coil - it would act like a transformer!

The current that is created in the ring creates its own magnetic field, but this magnetic field opposes the field in the iron rod (Lenzs law) causing the ring to be repelled (upwards).