This is a classic demonstration which involves what I would call a pickled
gherkin, many of you might just call a pickle. The demonstration involves taking
two electrically conducting pieces of metal, say forks and sticking them into
each end of the gherkin and then applying a high voltage across it.
Now, I must say immediately this is potentially an extremely dangerous experiment and
one you shouldn't try because if you touch the metal you will get a huge shock.
Neil had a historic piece of apparatus which we used. Unfortunately the plastic
safety cover had been lost, but because we were using it under controlled
conditions, it was safe. I found the piece of equipment about to be thrown out and
I wrote a note to Neil saying: 'save it for periodic videos'. Probably we should go
straight to watching what happens.
The first thing to realize is that when you pickle something you put it into a
mixture of salt and vinegar. Salt contains ions, sodium ions, chloride ions
and salt solution is electrically conducting but it's not very
electrically conducting compared to metal. So what you've got, our two
electrical conductors with a resistance in between and when you pass
a current through it, the resistor will heat up.
What you have here and you have to understand that a gherkin is a biological specimen. And biological
specimens are different, so it's not like doing a chemical experiment where the
chemical should be exactly reproducible as you can see this gherkin is
completely different from the other ones, not even as long. So you can't expect
everything to be totally reproducible.
What happens is that when you switch on the current, to begin with,
you see nothing and then at one of the ends and it's important it's at the end
because the maximum resistance appears to be where the fork goes into the
Gherkin, because there's less opportunity for the electricity to be conducted.
That heats up and it heats up enough for the sodium ions to give out light, in the
same way that you may have seen from sodium street lights, now what is
interesting is because mains electricity in most countries is so-called
alternating current, that it switches from positive to negative, positive to
negative. So the polarity switches around going through zero in the UK it's at 50
times a second in the States it's at 60 times a second. And if you watched the
gherkin under high-speed you can see that the light flashes on and off as the
current changes so if you think about the voltage this site is the so-called
neutral which stays at zero volts and this side goes up and down to + 200
volts down minus 200 volts, up and down, and the first time we saw it
there was a strong glow from the neutral end.
And we spent quite a lot of time thinking, why should it be at the neutral
end, rather than where the voltage is changing. So in the end we thought we'd
just try it again. And the next gherkin it went at the other end, so it's clearly chance.
If you look at the poor gherkin afterwards, it's a bit shriveled.
And the end where the light came out is charred.
We've actually got hot enough to burn the gherkin. You may know what temperature
gherkins burn, but we didn't.
So we decided to use a thermal-imaging camera.
The first thing that I noticed is you can see the steam coming out of the
gherkin, because the steam is hot and therefore is picked up by the camera,
whereas our camera barely noticed it in the dark. You can see that it really is
heating up and you can see that the end where the light is coming from gets much
hotter than the other end. We then thought that it would be interesting to
try and increase the resistance of the middle of the gherkin, so Neil cut out a
piece from the middle of the Gherkin like this. Our hypothesis was that if we made
the Gherkin narrow at that point, that part of the Gherkin should get hotter.
Because it's thinner it will conduct the electricity less well, so it has
higher resistance and the heating effect is related to both the current and the
resistance. And the current going through the whole gherkin must be the same, but
the current density will be higher in this small point and it'll get hotter.
And in fact what we found when we looked with the thermal-imaging camera
indeed it did get very hot there. Interestingly even where Neil had
cut it it didn't get hot enough to give any emission from the sodium, but you may
remember we got quite excited why it went at one end rather than the other, and
quite by chance in this experiment what happened was that one end lit up and
then after a bit the other end lit up. For a few moments both were glowing and
then it went to the other end. And you can see really well with the
thermal-imaging camera that the forks themselves, the prongs of the fork, get
very hot. Because they're conducting away the heat from this hot zone.
I have no idea why it changed from one end to the other.
I'm not an expert on gherkin structure and anyway it's probably pure chance.
But you'd have to do lots and lots more gherkins to be sure, when you look at
these experiments and you can see there are videos of this on youtube, you should
always think what is the science behind it.
They're great demonstrations but so often they're wasted, people just take it
as a joke and don't think about the physics or the chemistry behind it.
[laughs]
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