January 12th in 1832 Michael Faraday did the following experiment in London:

He placed two copperplates in the Thames river at opposite ends of a bridge crossing at 300m apart. He then connected these copperplates with insulated wires to a voltage meter.
He measured a 19mV voltage. Magnetic field in London is measured to be 43µT (43*10^-6 T) and inclination is 71 degrees.

Why did Faraday's meter measure a voltage and what was the velocity of the water in the river Thames at that time?

An electric charge moving in a magnetic field is affected by the Lorentz force which acts perpendicular to its velocity and the magnetic field.  So charged particles moving in or with the water will get diverted to different sides depending on the sign of their charge and induce a voltage across the river.

What I am not sure about is what exactly the relevant charged particles are.  They might be ions in the water (most probable, I think - water = 2 H2O can dissociate into  H3O(+) and OH(-) ).  Other possibilities might be the electrons moving slightly to one side and the atom nuclei to the other, or water molecules, which are dipoles, aligning in a way to create a voltage.

Posted by vswitchs on 2006-10-20 16:25:36