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?

To gregg:

Good work!

BTW: one only calls it the Hall effect if it happens to electrons in a solid.

To Larry:

The positive and negative ions don't cancel out because they are diverted by the magnetic field to opposite directions.  That's what builds up the voltage.

The magnetic poles of the Earth are very close to the geographic poles and hence to the rotation axis, so I'd guess the magnetic field doesn't change much during rotation at London latitude.

To brianjn:

Water always forms a few ions, even chemically pure water.  The proportion of ions is of course small; 2.8 x 10^-9 at 37 Celsius according to this web page:
http://www.lsbu.ac.uk/water/ionis.html

If you rotated the plates to be placed upstream and downstream (I think that's what you meant), there would be no voltage, because the Lorentz force is perpendicular to the ions' velocity. So there is only a voltage across the river, not along it.

Posted by vswitchs on 2006-10-22 12:29:28