Candles at Christmas Revisited

Last week I gave a presentation at NPL on the physics of candles.

Above is a video of the slightly chaotic 32 minute presentation and if you are so inclined, you can download the PowerPoint file here.

I have spoken before about the wonderful physics of candles. But in revisiting the subject I learned that wax is basically not flammable and I felt obliged to mention this inconvenient truth!

The highlight of the talk is using a candle to power a thermo-electric generator, which in turn powers a USB port, which in turn powers a torch, which is brighter than the candle.

Sadly, in the rush at the end of the talk I forgot to actually measure the brightness of the torch!

Next time!

And by the way, here is the slow-motion candle-relighting movie that is embedded in the PowerPoint but which doesn’t show up well in the lecture theatre view.

Thanks to Brian Madzima for the videography and editing, and Nikita Mezhnyakov for the photograph.

Cold Candles

Do candles which are first chilled in the freezer burn longer than candles started at room temperature? No. I lit the candle on the right after taking it straight out of the freezer.

Please forgive this last piece of candle nonsense! After the talk last week the first questioner asked me:

“Why do candles burn for longer after you put them in the freezer?”

Frankly it wasn’t a question I was expecting. I did have the presence of mind to ask if she had compared the cold candles with some she had kept at room temperature.

She said, “No”, but she did say that when she took the candles out of the freezer they had “burned all evening”.

Faced with this experimental ‘fact’ I speculated that perhaps the flame lost some energy melting the colder wax. This seemed to satisfy the questioner, but I was not happy.

So the next night I put a candle in the freezer and after a couple of hours I burned it with a similar candle I had kept at room temperature: I could see no obvious difference.

And so this evening I conducted the definitive experiment. I had previously chilled a candle in the freezer and my (uncalibrated*) thermometer told me its temperature was -13.8 °C. The comparison candle was at room temperature: +18.4 °C. These temperatures should be compared with the melting temperature of wax which is typically 55 °C.

Both candles initially weighed 54 grams and I lit them quickly to maximise any cooling effect before the ‘cold candle’ warmed up.

Making measurements in the usual way  it became apparent that the candles burned at nearly the same rate, and if anything, the ‘cold candle’ burned a little faster!

The three gram difference is only just resolvable with my weighing machine, but it corresponds to nearly 9 mm difference in height and as the photograph at the top of the page shows, the ‘cold candle’ is definitely shorter than the ‘warm candle’.

Graph showing the mass loss versus burning time for a ‘Cold Candle’ taken straight from the freezer and a ‘Warm Candle’ taken out of a drawer. Although the difference is not large, the cold candle has burned faster than the warm candle. (Click for larger graph)

So what, I hear you silently wondering, is the point? The point is that I now know the answer to this question! And so do you. Experimental physics has answered one question and – as it frequently does – raised several more. For example: is the difference in burning rates really associated with temperature or was there some other variable I didn’t control. Perhaps another happy evening of experimentation will resolve this!

I love Experimental Physics, and when work is depressing or unsatisfying, the ability to definitively resolve such questions as this – pointless as they are – gives me considerable comfort.

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*Sorry: At home I have to operate to much lower standards 😦

Candles at Christmas

My colleagues at NPL have just finished making a video of my talk about candles so I thought I would share it with you while it was fresh.

Thanks to Lloyd for the video and thanks to everyone who helped with the talk. Personally I hate watching myself, but I hope you enjoy it.

You can follow up on the topics in the talk with the links below:

• This is my post about candles made out of spermaceti
• This is my post about weighing the candles and measuring their heat output

Looking on the bright side

A parliamentary ‘standard candle’ – made from spermaceti – a substance found only in the heads of sperm whales. Click for a larger view.

I gave a 20-minute talk last Thursday – a Christmas talk about candles. As usual with these things, I started out knowing a little, but the process of preparing the talk involved learning lots of interesting things. And then not mentioning them.

I spoke to colleagues all over NPL to ask for help: one lent me a precision balance to weigh a candle as it burned, and another built me a device to power an electric torch from a candle! I thought that was very cool.

When I asked my colleagues in the optical team about measuring the spectrum of light from a candle their eyes lit up and I could barely stop them talking – they knew so much.

After lending me a spectrometer, they mentioned that they had an old ‘standard candle’ in their office. In the ‘old days’ it was a candle such as the one in the picture at the top that formed humanity’s standard for ‘an amount of light’.

And even though our modern standard  – the candela – is defined quite differently, its magnitude can still be linked back to the amount of light given off by a standard candle.

Gazing at the candle I was astounded. Had a whale really been killed in order to make this candle? The answer was ‘Yes’: I felt like I was holding ivory in my hands.

The idea that we would kill whales in order to extract oil and make candles is now so bonkers that we can hardly credit it. And this made me feel a little better.

It made me realise how desperate people must have been for light, and that for all its faults, our civilisation has now all but solved this problem. And that made me smile – and momentarily reflect on the brighter side of humanity’s adventure with energy.