Swans on Tea

Free charges in conductors

There’s a great discussion in Griffiths book on electromagnetism (typical book used for junior/senior course for physics majors) about what happens to free charges inside a conductor. He discusses how they will always find their way to the surface, distributing themselves to cancel the field inside the conductor. If they don’t do that, there will be a residual field to push more charges around. This happens until the field is totally cancelled. In a footnote he mentions how this surface effect only happens in 3D. In lower dimensions, like charges on a plate, say, the charges don’t necessarily go to the boundary.

Some neat simulations of both 3-D and 2-D cases.

A time-lapse taken from the front of the International Space Station as it orbits our planet at night.

via The Bad Astronomer, who makes a recommendation

Make sure you set it to the highest resolution, then make it full screen. You’re welcome.

Enigma machine to go under the hammer

A version of the three rotor Enigma machine — used by the German military to encrypt messages, the code of which was subsequently cracked by a team at the legendary Bletchley Park complex — will be auctioned at Christie’s on September 29.

The last one sold for more than $100k, which is just a bit outside of my price range.