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!