09 April 2004

More on the "Hot Chocolate" Effect

Dear Bob Hall,

The observation you have made is well known. Sir Wm. Bragg noted it many years ago, and F. Crawford wrote a brief paper about it in the American Journal of Physics some 25 years ago. Despite it being a relatively well known phenomenon, I suspect that there is still much to learn about it. For example, Crawford explained the phenomenon as a systematic variation of wave speed in the coffee during the time interval that it clears of the small bubbles introduced during stirring. While the wave speed does vary as Crawford describes, I am not convinced that this fully explains the phenomenon. In particular, there is also a systematic variation of Q in the system comprised of coffee and cup as the bubbles clear, and this is quite capable of producing the effect, or something similar. I made a series of measurements that suggest this is so. An amateur scientist could turn this into a fruitful study.

You can find references to the work of Crawford and Bragg, and read also about the experiments I carried out, at the following URL.

http://www.kilty.com/coffee.htm

Regards,

Kevin T. Kilty

"Anti-gravity" toys?

Good Day,

I am looking for plans to build one of the Revolution "anti-gravity" toys (it is the magnetically levitated top on it's side). The specific plans I am after show this device with solar cells on the spindle, so that it will spin by itself when under a light source. I would appreciate any leads your or your readers can provide.

Best Regards,

Jim DeRosa
jderosa@zoominternet.net