How Smoke Detectors Work

27 March 2005

Interview with

Philippa Law interviews Dr Lucy Green & Dr Bill O'Neill


Philippa - Smoke alarms save loads of people's lives every year, and this week I've been finding out how they work.

Lucy - Hi, I'm Lucy Green from Cardiff University. There are two different kinds of smoke detectors. The first one is called an ionisation detector. This uses a radioactive source within your smoke detector, although it is very very small.

Philippa - Let's speak to Bill O'Neill from the Institute of Manufacturing at Cambridge University.

Bill - The chamber itself is quite simple. It consists of two metal plates with a voltage across them, which we apply with a battery along with a source of ionising radiation. The Americium-241 sits under one of the plates and it emits alpha particles. Alpha particles have the following properties: they essentially ionise the nitrogen and oxygen atoms that are sitting in the chamber. Ionise in this case means that it knocks an electron off an atom so you end up with a free electron and positive ions. What happens is that the negative electron is attracted to the plate with a positive charge, and the positive atoms are attracted to the plate with a negative voltage. Essentially the electronics in the smoke detector sense these small amounts of electrical current that these electrons and ions generate as they move towards their plates.

Philippa - But what happens when smoke gets into your chamber?

Lucy - Well the smoke actually attaches itself to the ions and it stops the current from flowing.

Bill - The smoke detector sees this drop of current between the plates and sets off the alarm.

Philippa - That makes sense, but what's this about radioactive material in them? Surely that's dangerous?

Bill - No. Whenever you hear the words 'nuclear radiation' it sets off alarms in people's minds anyway. But if you think about the amount of radiation in a smoke detector, it's extremely small. It's predominantly alpha radiation, which can't even penetrate a piece of paper, let alone damage anyone sitting beneath it.

Philippa - That doesn't sound so bad. Now, does Einstein have anything to do with all this?

Bill - His work on the wave duality of photons and the equivalence principle of energy and mass allowed people like Ernest Rutherford to develop his work on the alpha particle and Thomson on the electron. So many of the early atomic physicists were really working in the dark and Einstein helped illuminate their environment slightly.

Philippa - Ionising smoke detectors are the cheapest and most reliable smoke detectors you can get. But there's another kind too.

Lucy - The second kind of smoke detector is called the photoelectric detector.

Philippa - Ah, photoelectric, like the photoelectric effect!

Lucy - That's right. This is using the theories Einstein laid down for us. In this kind of detector, you use a light emitter and a light receiver. When you have your light receiver we know from the photoelectric effect that when photons fall onto your receiver you can generate a current. So in your photoelectric detector you have a beam of light and when the light falls onto a surface you have a current generated. But what happens when smoke gets into the chamber? Well it can move in front of the beam of light and stop it form getting to your detector. The current is broken again and the alarm goes off.

Philippa - Since 1992, all new homes have been fitted with interlinked smoke detectors, which means that if one alarms goes off, then that sends a signal to other alarms in the house and they all go off too. Building on that technology, there's now a company in Kent who have invented a new system of smoke alarms that signal to each other via the mains electricity in the home. That means that not only do you not have to fit a whole load of new wires in your house to get one of these safer systems, but also the smoke detectors can actually turn off your electricity. So say if the toaster is puffing out lots of smoke and is about to catch fire, the detectors detect the smoke and switch the toaster off so it can't get any hotter and start a fire. Just like the other things we've heard about in the last few weeks (the CD player, the calculator, the remote control) the technology in smoke detectors may not have been invented so soon, or in the same way, were it not for Einstein and the work he was doing 100 years ago. So as you're listening to your CDs or changing channels on the TV, just remember that Einstein isn't only famous for his silly hair. He's had a tangible influence on all our lives.


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