At 10:52 AM 6/20/2011, Jed Rothwell wrote:
Abd ul-Rahman Lomax wrote:
There is a classic demonstration, used to be
common in high school physics labs: you boil
water in a paper cup, over a flame, as I recall.
A paper cup!?
Yeah. I think one of my high school science
teachers, the chemistry teacher as I recall, had
a contract with Dixie to come up with science
experiments using Dixie Cups, and this was one of
them. There are lots of pages on the internet on
this,
http://boards.straightdope.com/sdmb/archive/index.php/t-50249.html
is one that discusses it, with some experimental reports.
Please, folks, don't stick your hand in that
invisible steam. It may only be at 100 degrees,
but it's dangerous, it's carrying a lot of
heat, which it will cheerfully transfer to your
skin, in a flash. Maybe if you are *fast*, you
wouldn't get burned, but I wouldn't advise trying it.
Neither would I, but that is what I have seen
grizzled boiler room workers do. They don't hold
their hands in the steam! They wave their hand
through, quickly. The way a person can wave a
finger through a the flame of a candle. If the
steam is wet, droplets adhere to the skin and that hurts.
They do not do this with superheated steam,
obviously. Only process steam a little above 100°C.
I have my own method, as well, of determining if
a metal surface has possibly dangerous 60 cycle
AC eletrical leakage currents. This will detect
even pretty low leakage, below the dangerous
level. I make sure my shoes aren't wet, nor am I
touching any grounded metal, and I run the back
of my fingers over the metal surface. If there is
leakage, I can feel the vibration.
Used to be common with electrical appliances with
no ground plug. Turn the plug around, it usually
eliminated the effect. With polarized plugs,
assuming that people wired the socket correctly,
this became a non-problem. (except for those
badly-wired sockets!). With grounding plugs,
again, not a problem, assuming the ground is connected!
It's nice to know about the perceived vibration
effect (it's probably due to electrical
stimulation in the muscles of the fingers, just
enough to be perceivable. If there is a dead
short to a hot wire, it's still not painful. Long
as I don't touch a ground somewhere!)
