Neal our technician suddenly got
inspiration from his old physics teacher
he's devised a machine which will allow
him to put a high current through a
piece of thin wire and as he turns the
knob the current goes up and up and up
until the wire fails no I'm ready when
you are for this one so we started with
copper what happens is that as you put
the current through the copper he gets
hotter and hotter and hotter and in this
case copper melts before it catches fire
so suddenly you see the wire sagging
down and down and down and eventually it
breaks and as it breaks there is a flash
as the copper vaporizes and you may
remember that copper gives a nice green
color in the vapor so you see a very
quick flash of green light
so Neil had a great ambition to try and
burn aluminium so he got piece of
aluminium wire and heated it up but like
copper aluminium melts before it catches
fire in air so suddenly you'd start
sagging sagging sagging and after a
little bit its breaks Neil was a bit
disappointed nothing burned
so we tried magnesium didn't have
magnesium wire so we tried magnesium
ribbon that's relatively flat material
and as you heat that because the heated
metal expands the magnesium started
arching upwards and then suddenly it
drooped there as probably many of you
know that once you set fire to magnesium
it keeps burning so the whole lot burnt
away with clouds of white sort of smoke
which essentially is finely divided
magnesium oxide
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magnesium is a metal that once you light
it it generates so much heat that the
burning becomes self-sustaining it's
probably because the metal doesn't
conduct away the heat very much from the
area that's burning
whereas aluminium and copper if you set
it on fire and remove the source of heat
it quickly goes out probably the metal
just cools down too fast
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Neal suddenly remembered he had some
special wire he took Brady to his
special store he calls it the dark hole
doesn't want me going there and might
start taking things from it in the dark
hole was the remains of a
superconducting magnet a very powerful
magnet that was used in an instrument
some years ago the magnet consists of a
coil and Neil cut off a piece of this
alloy the wire that's used for these
magnets has to be insulated otherwise it
would short-circuit and it's insulated
with varnish so as it heats up there's a
bit of smoking as the varnish vaporizes
slightly disappointingly it behaves much
the same as the other metals
SAG's flashes and breaks
next Neil produced a piece of tungsten
wire tungsten wire is what's used in
old-fashioned lightbulbs it was really
interesting as you heat it up it gets
hotter and hotter tungsten has the
highest melting point of any of the
metals that we used and it starts to
burn doesn't melt at all and just gets
thinner and thinner and thinner until it
fails he ended up with quite a
terrifying looking spike a really narrow
tungsten so professor this did fail
quite quickly but light bulbs don't fail
quickly ideally what's the difference
between this and a light bulb the
difference from the light bulb is that
in a light bulb the tungsten is not as
hot as at the end of Neil's experiment
and of course the bulb itself is filled
with argon gas rather than with air Neil
and I were both really quite fascinated
by this very narrow spike that is made
and I think it is because one part of
the wire must have started getting
thinner than the rest and therefore
hotter and therefore preferentially the
hottest part will have burnt fastest
Brady really wanted to do gold Neil was
a bit reticent gold is expensive so he
put a glass dish underneath now I must
say that gold was probably the most
disappointing of them all because it
heated up it melted it broke
it didn't give a really very convincing
flash Gold doesn't have a beautiful
green color like copper but Brady was
satisfied
Anil didn't lose any gold I think the
most exciting metal and this was a real
surprise was molybdenum when we heated
up molybdenum even when I was watching
the camera from the distance there
seemed to be some sort of pulsating on
the picture
and when we looked at the metal
afterwards it was extraordinary
because instead of having a smooth wire
there were little blobs of molybdenum
metal all the way along the wire and
these blobs were really quite evenly
spaced
now it turns out that this phenomenon
has been seen in exploding wires using
very high speed cameras
I think it's unusual i and none of my
colleagues had seen like anything like
this before
now you probably or some of you who
specialist in this area probably have
some really good explanation of why this
happened I think what happened is that
the molybdenum ëtil began to melt on the
surface of the wire so you imagine the
wire covered with a thin layer of liquid
and because liquid has surface tension
this is why liquid forms drops I think
that the liquid separated into little
drops because the surface tension pulled
it together and so you get what would
normally be little drops of liquid but
they solidified so we've somehow got
them fossilized along this wire all my
colleagues that I've shown these bits of
wire to have got really excited it's
really surprising that you can get such
an even spacing I did a bit of research
on this phenomenon and I came across a
word I'd never heard before called
angeloids angeloids
is the word that mathematicians use to
describe this sort of object I would be
very interested to hear from any of you
who has a really good explanation of
what happened Neil thinks that the
spacing might be related to the
frequency of the mains so he's keen to
try and use direct current rather than
alternating current and see whether we
get the same effect I think there'll be
no difference I had the idea that we
should turn the apparatus through 90
degrees so the wire is vertical and it
would be much more difficult for it to
sag and what we found was indeed it
didn't sag so much at least until it
break and we got a much better shot of
the flesh of
green light as the cup of vaporized
there was still slight bending but it
was much less than before just as we
were packing up I persuaded Neel to make
a coil of copper we put this coil
vertically my idea was that the coil
would be floppier than the straight
piece of wire so it might sink more
but then it collapsed quite
spectacularly once it's broken
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but I think chemically what was
interesting to me was that if you looked
at the bit that was broken you could see
the black copper to oxide on the surface
of the copper showing that there was at
least some reaction with oxygen but very
much less than saying the burning
magnesium I was really grateful to Neil
because he devised a demonstration which
almost turned into an experiment which
made me think I never thought that piece
of molybdenum wire could be in the least
bit interesting and Here I am
the day after Neil did the experiment
still thinking about it and anything
that makes scientists think is really
good and anything that makes us think
differently is brilliant
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