Friday, 15 August 2008

Join the Jet Set!

Gosh - it's been nearly a whole year since my last post. Gulp. What have I been up to? Well, I'm nearly at the end of my first book (my publishers won't be happy that if they find that I've been writing this instead of the final chapter...) Things have been incredibly busy on the tutoring and training front as schools in the UK gear themselves for the new 14 to 19 Diplomas...

So, that's the excuses over with. What about exciting experiments? Here is one that Jess and I attempted this Wednesday - the jam jar jet. Except we didn't have a jam jar. So we had to make do with a beer bottle. And it all got a bit explosive. Here's how it went...


When Francois Reynst was a young boy he discovered that if you poked a hole in the lid of a glass jar, put a small amount of neat alcohol in the bottom, let the alcohol exapourate a little, and then light the top of the jar, flames shoot out of the top of the jar and then get sucked back into the jar again. Then flames get ejected out of the jar. And then get sucked back in again. And this cycle repeats until all the fuel is used up. In this post I tell you how one of my students and I tried to reproduce Reynst's experiment. Many thanks to Jess for helping me out with this one.


We didn't have a jar. And we didn't have any alcohol. We had:

  1. lighter fluid
  2. matches
  3. assorted glass bottles (from the recycling bin)
  4. blue tac

What we needed was a vessel that would allow the air and lighter fluid vapour to circulate easily inside it. After careful consideration and detailed mathematical analysis (ok, I squinted one eye and guessed) we decided on a small beer bottle, which had a fairly large cross-section compared to its height (see picture - courtesy of Jess). What we are about to make is tchnically called a Reynst Combustor...


  1. Squirt a little lighter fluid into the bottle. The amount you need is really a matter of trial and error. A covering across the bottom of the bottle around 3mm deep seemed to work best.
  2. Seal the top of the bottle with blue tack.
  3. Put bottle in the freezer for 5-10 minutes.
  4. Take bottle out of freezer and shake vigourously.
  5. Unseal the bottle. It should hiss as air is drawn into the bottle. Light the top of the bottle now.
  6. The inside of the bottle burns with a vibrant blue glow for around 5 seconds and howls as it draws air into the bottle.

*DON'T* use a match or a lighter to light the top of the bottle in case it explodes (see the end of this post)

How it works

Air is drawn into the bottle and mixes with the lighter fluid vapour. When the vapour is ignited a burst of hot gas is ejected from the top of the bottle. This leaves a slight vaccuum inside the bottle which then sucks air back into the bottle. More lighter fluid vapour mixes with that air and, because it's hot in there, the mixture re-ignites and the cycle begins again. Besides being called a Reynst combustor, this is also called a relaxation oscillator.

How it doesn't work

If you get the ratio of oxygen to vapour wrong it does one of two things:
  • nothing
  • it explodes
We didn't manage to catch on film the jet working well, and there plenty of times when it didn't work at all. But we did record it exploding...

Further investigation

Check out YouTube for more videos of homemade pulse jets (that worked). The pulse jet was the engine that powered the Nazi V-1 rocket - called a "doodlebug" because of the distinctive sound a pulse jet makes...

Also check out the following web resources...

Thursday, 15 November 2007

DIY Hydrogen Plant

In this post we'll be looking at a way of making hydrogen from some basic kitchen ingredients. Many thanks to Aston for helping me out with this project.

You will need...
  • Aluminium foil
  • Caustic soda. Check the packet and make sure you are buying sodium hydroxide.
  • A glass bottle. Glass is far better than plastic as it is going to get very hot.
  • Rubber gloves
  • Old clothes
  • A garden
  • Party balloons

1) Change into your old clothes and put on the rubber gloves. Caustic soda is nasty stuff and you don't want to get it over yourself.
2) Fill the glass bottle around a third full with cold water.
3) Add caustic soda crystals a little at a time. Shake the bottle every time you add more crystals to dissolve them. When the water starts to become cloudy and there are crystals left in the bottom of the bottle stop adding - the water has become saturated and there isn't any point putting any more in. Feel the bottom of the bottle. Does it feel warm?
4) Make some aluminium foil torpedoes that you can drop into the neck of the bottle.
5) Drop some torpedoes into the bottle, give the bottle a swirl and then wait for a few

You should start to see the aluminium start to fix and bubble. The water will get very hot and may well start to boil. You should see wisps of steam coming out of the top of the bottle.

If you have a balloon handy then pull it over the top of the bottle and watch it inflate...

Tie off the balloon and let it go. It floats away!

The balloon is full of hydrogen. Apart from being lighter than air hydrogen has another interesting property to all of us kitchen experimenters - it's highly explosive!

As the reaction is going on, put a lit match to the top of the bottle. What happens?

How does that work then?

Sodium hydroxide and aluminium react very strongly with each other. So why the slight delay before the reaction starts in earnest? That's because the foil will be coated with a thin layer of oxide, which the soda has to react with before we get to the reaction with the aluminium proper...

2Al(s) + 6NaOH(aq) → 3H2(g) + 2Na3AlO3(aq)

For the non-chemically minded, that's lots of hydrogen - H2(g)!

The reaction between aluminium and caustic soda is exothermic. This means that it gives out (a lot of) heat.

And remember: hydrogen filled balloons may be fun to play with, but this is the sort of thing that can happen if you are not too careful...

Thursday, 4 October 2007

The Acid Test

In this experiment we are going to be testing for acids and alkalis using nothing more than a red cabbage. Thanks to Aston for helping out with this one.

We need a home-made way of testing for acids and alkalis - a bit of kit otherwise known as a litmus indicator, or pH indicator. Have you used litmus paper at school? In acids the paper turns red and in alkalis it turns blue. Our home-made tester is slightly different - it turns pink in an acid and a bluey-green colour in an alkali.


The first step is to rip apart a red cabbage and put all the bits of leaves into a large saucepan. Pour boiling water into the saucepan over the leaves and leave it to steep (like a pot of tea) for a few minutes until the water is nice and purple.

Now pour off the purple cabbage water into separate glasses and we are now ready to test for acids and alkalis.

Finding the ingredients...

Acids are usually used in cooking: vinegar is an easy acid to get hold of. Lemon juice is also acidic. We tried lemon juice and vinegar. We tried putting the same amount of lemon juice in the indicator as vinegar. Our experiment seemed to show that lemon juice is more acidic than vinegar... and raspberry juice...

...which was very mildly acidic - which means it could well give you a poorly tummy if you drank too much of it - and also shop-bought coleslaw, which also contains a lot of vinegar but strangely didn't turn our indicator pink. We decided that perhaps the added "acidity regulator" ingredient had kept the acid levels down somehow.

Alkalis are usually used in cleaning (the old fashioned name for "alkali" is caustic, as in caustic soda). We tried baking soda (used in cooking but also a very weak alkali - the weakest alkali amongst the sodium compounds, if you didn't know), a Rennie (an indigestion tablet), bleach and a dishwasher tablet. All turned the indicator a murky blue to varying degrees, but the dishwasher tablet had the most marked effect - turning the indicator a yellow/green colour. It was difficult to tell if the change in colour is due to the fact that the detergent in the tablet is caustic or there is some other sort of chemical reaction going on...

So why would you want to test for acids and alkalis?

Okay, generally you probably wouldn't, but one place where the test is used quite a lot is in gardening. Certain plants like an acid soil while others love an alkali soil. A litmus test tells you if you have got your soil just right for your plant. My grandparents used to have a hydrangea in their front garden that they used to change the colour of the flowers of by mixing different things into the soil - an acid soil makes them pink and a neutral soil (mixing lime into the soil takes away its acidity) makes them purple. Don't laugh: that's the sort of thing that passed as entertainment when I was growing up in the 1970s.

Have a play with other chemicals but remember: cleaning chemicals can be very dangerous so don't let them splash into your face or get mixed up with anything you are planning on eating later.

Happy experimenting!

Thursday, 30 August 2007


I'm taking a bit of a break from experimenting this week as it's the bank holiday in the UK and everyone is away (apart from me - boo hoo). I've been thinking about the YourMathsTutor website and finding a neat way of integrating some sort of LaTeX maths editor into the forum to make it easier for people to include extracts of maths in their posts. I'm a Moodler, amongst other things (yes - it is legal. Click here for more information). And being a member of the Moodle Mathematical Tools forum I came across the new DragMath tool from Chris Sangwin and Alex Billingsley at The University of Birmingham here in the UK. I thought it would be marvellous to incorporate this tool into the YourMathsTutor forum. To view the results of an afternoon's sawing and hammering noises coming from the forum corner of the YourMathsTutor site click here.

The YourMathsTutor forum is actually an SMF (which stands for Simple Machines Forum) forum, a pretty powerful open source forum which has only been going a few years but even so is becoming extremely popular.

If you are interested in how I integrated DragMath into my forum then you can download a tar.gz archive with all the bits you need (and a readme) from here. Maybe there is a better way of doing it? If so then please do post a comment. I think DragMath is an excellent bit of work and would love to see it take off.

Wednesday, 15 August 2007

The Singing PC

I'm part way through attempting to knock together an experimental setup I saw many moons ago on a visit to the University Of Hull. The idea is pretty simple: convert sound to light, shine the light onto a detector, then finally convert the light back into sound again. This one is quite interesting from the - possibly boring, sorry :'( - physics point of view, because we are having to deal with different types of energy...

  • Sound
    Sound is a pressure wave - a longitudinal wave - of moving air molecules. I have often read in text books and on the Net that sound is energy but that isn't right. Because sound is air moving about it is technically kinetic energy.
  • Electricity
    This is a complicated one. The concept is quite simple:- all atoms contain electrons and in electrical conductors the electrons are free to move from one atom to another (in electrical insulators the electrons can't move so they don't conduct electricity). Actually how it works is a bit more complicated (involving magnetic fields and things). So although electricity is moving electrons this isn't simply kinetic energy - there is more to it than that.
  • Light
    This is electromagnetic energy - a mixture of electrical and magnetic energy.

We are going to transform energy into these three different types of energy...


Initially we will put together the light-to-sound converter. Here is the parts list (click on the part to jump to the relevant Maplin page)...

  1. Photodiode SFH2030
  2. Mono preamplifier
  3. Norwich 3.5W audio amplifier
  4. 49mm Mylar speaker

...and then there'll be wire, batteries, and all that, to power the thing. Note that the amp and preamp draw about 500mA (according to their spec.) so I powered this with 6xAA batteries (rechargeable, of course!).

At this stage I should admit that I was going to build my own preamp and amp but when I was wondering around Maplin looking for parts and found these I was overcome with a bad bout of idleness on my part. That said, I know that you can make a photo detector out of a piece of heated copper and a drop of brine - an idea for a later project there, I think.

Here is a schematic (click on the image to see an enlargement)...

And here is a picture of the prototype (on my particularly messy desk)...

Your eyes don't see continuous images - in other words what you actually see flickers, but you don't notice it (what you see is also, bizarrely, upside down. Your brain flips the image on the back of your eye the right way up). Your ears have much better resolution, is the term. So when I switched on the light over my desk it buzzed. That's because mains electricity is supplied at 50Hz (50 cycles per second). This means the bulb will be flickering 100 times a second - I can't see that but I can hear it with my new new detector.

I was having a play with the monitor refresh rate on the PC when my eldest lad came in and suggested that we have a look at a Linux project called Tempest For Eliza. This program uses your monitor to broadcast music on an AM radio signal. It does this by flickering the screen, causing your monitor to radiate radio waves that you can pick up on a radio (here is a YouTube video of this). Our idea was that our light detector should be able to pick up that flicker and turn it, in theory, into music.

Here are the results...

Wednesday, 8 August 2007

Lemon powered calculator!

In this project we are going to build a lemon battery, then use this to power a calculator. Many thanks to Jess for helping me with this one.

With a zinc and a copper electrode stuck into either side of a juicy lemon you can generate about 0.9 volts. Connect a couple of these lemon 'cells' together and you can make a battery that can generate enough power to supply a watch or a calculator.

What you will need
  1. two galvanised screws (must be zinc coated - that's what galvanised means).
  2. high current single core copper wire (not the stranded type). My wire was an offcut from Homebase.
  3. stranded wire (from the Worcestershire Resource Exchange).
  4. a cheap calculator (mine was £1.49 from Staples).
  5. 2 lemons. Have a think about going organic. I know they are more expensive but by buying organic you are not promoting the use of nasty chemicals (they do less to them but they are more expensive. Erm...).
  6. plastic cups (to hold the lemons).
  7. piece of wood (to attach everything to).
  8. wood screws and cup washers (to do the attaching).
You might also find a voltmeter useful to check the voltage your lemon battery is generating and the polarity (in other words which terminal is positive and which is negative). We also used a soldering iron to connect all the wires together but it isn't vital as long as you've got a good connection at each joint.

How to put it all together

Firstly cut the tops off the plastic cups so that they hold the lemons in place. Screw these onto the wood, nearer what will be the back.

Next cut yourself 2 bare lengths of copper wire about 6 or 7 cm long.

For each lemon, push a screw into one side and a length of copper into the other. The copper will be the positive terminal and the screw the negative terminal (if you have a voltmeter you can check this).

Now you need to connect the negative terminal of one cell to the positive terminal of the other (in other words connect the screw in one lemon to the copper wire in the other). We soldered a length of wire between them, but it really doesn't matter how you join them up as long as you have a good connection.

Now solder a longer wires to the two ends of the battery ready to connect to the calculator.

The calculator we have is solar powered (see the post Charge your batteries using the sun! for more info on solar cells) with a battery backup. We disconnected the solar cell and soldered the wires from our battery onto the calculator's battery terminals (if you see what I mean)...

Now try turning on your calculator! To prove to yourself that it is the lemon battery powering the calculator try pulling out one of the copper electrodes.

How does that work then?

All metals try to either give out or take in electrons. So to create a battery firstly we need two metals - one that gives out electrons and one that takes them in. We used zinc (the coating on the screw) and copper (the wire) because they are pretty easy to come by. If you put the wire and the screw into a liquid that will conduct electricity, technically called an electrolyte (in our case lemon juice), then electrons from the copper (which tries to give out electrons) will move over to the zinc (which tries to take in electrons). Electrons are negatively charged so the copper, which is now short of electrons, ends up being positively charged and the screw, which now has electrons from the copper, ends up being negatively charged.

Try putting a screw on one side of your tongue and a length of copper wire on the other. Does it tingle?

Thursday, 2 August 2007

A Prisoner Of War Radio

Here is a clever experiment. It is a called a foxhole radio (US soldiers call dugouts 'foxholes') or a PoW radio (i.e. prisoner of war radio). If you were trapped in a prisoner of war camp, like R.G. Wells who was captured by the Japanese, then a vital skill was being able to knock up a radio out of anything you found around you. Prisoners of war often used a small piece of coke (the fuel, not the drink or the drug). Solders in the field used a Gillette Blue razor blade (see the picture, right). We are going to use a junior hacksaw blade from Homebase. It must be blued (when you hold the blade up to the light it has a blue'ish lustre to it). The blue comes from a heat treatment that is applied to the blade to make it stronger. We are also going to need...
  1. 30 SWG enamelled copper wire
  2. High impedance earpiece (or a piezoelectric speaker will do), but it must be high impedance - a crystal earpiece rather than a magnetic one.
  3. A kitchen roll
  4. A scrap piece of wood to attach everything to
  5. Short-length self-tapping screws
  6. Cup washers
  7. Paper clip
  8. Safety pin
  9. A short length of pencil lead
  10. Small piece of foil
  11. Sand paper or emery cloth
The cup washers are quite a neat way of joining two bits of wire together, as you will see in the picture below (click on the picture to enlarge it). Here is the finished set up (thanks to Jess for helping me put this one together)...

Assembly Instructions
  1. Wind 100-150 turns of our copper wire around the kitchen roll. Cut the spare, bare length of kitchen roll off as you don't need it and it will be in the way. Firmly attach this to the wood (Jess and I screwed it on).
  2. Using some sandpaper or emery cloth remove the enamel off the ends of the wire and rub the enamel off the copper wound around the roll along the length of the tube, as you can hopefully see in the diagram (click on the diagram to enlarge it)
  3. Rub some of the blue oxide off the end of the hacksaw blade ready for a wire to be attached to the bare metal (don't rub the oxide off all of it!)
  4. Attach the pencil lead to the point of the safety pin by wrapping foil tightly around them both. The lead is then "sprung loaded" against a blued bit of blade (that's why we are using a safety pin).
  5. Now join everything together using the schematic shown below (click on the image to see an enlargement)...

Wipe the paper clip against the exposed copper wrapped around the kitchen roll. If all is well you should be able to hear cracking from the earpiece. You should, in fact, be able to hear (probably) BBC Radio 5.

How does that work, then?
Our hacksaw blade is technically called a detector or diode. A diode allows electrons to flow one way but not the other. A coil and a capacitor together made an oscillator. Inside the oscillator electrons are actually doing the oscillating. If we adjust the position of the paper clip then we can make the electrons oscillate at different frequencies. That's how we can tune in this radio. The coil is obvious but where is the capacitor? Capacitors have two plates, and in our case the antenna is one plate and the ground beneath it is the other. The electrons are trying to oscillate back-and-forth, but the detector only allows the electrons to move one way. That means the electrons are shunted around the circuit, generating a current which is then converted to sound in the earpiece.

Now you are armed with this knowledge, if you are stuck in a dugout or a prisoner of war camp you should have a good idea of how to be able to put together a simple radio receiver for yourself!