**What it shows:**

The mathematical description of electromagnetic induction as formulated by Maxwell and Faraday requires two different sets of equations to calculate the induced voltage, depending on whether the coil is stationary and the magnet moving or vice versa. In fact, as this demonstration shows, the voltage is the same as predicted by the two sets of equations.

**How it works:**

The apparatus is identical to demonstration Faraday's Law, and is described in detail there. Briefly, it consists of a galvanometer hooked up to a solenoid; the magnetic flux through the solenoid can be changed by either plunging a bar magnet into the solenoid or moving the solenoid while keeping the bar magnet stationary. All things being equal, and assuming you manage to move the magnet or solenoid at the same rate, the induced emf and resulting current will also be equal in the two cases.

**Setting it up:**

The galvanometer and solenoid sit on the lecture bench. They are large enough to be seen by the audience without visual aids.

**Comments:**

So simple and so fundamentally important. Einstein reasoned that it made no sense to have two very different sets of equations to predict what ought to be, and is, the same result. In his famous paper of 1905 entitled "On the Electrodynamics of Moving Bodies," Einstein postulated that the physics of electromagnetic induction has to look the same whether viewed from the magnet or coil reference frame. ^{1 }

1 J.A. Wheeler, *A Journey into Gravity and Spacetime*, (Scientific American Library, NY, 1990)