D is for Detectors

By Siobhan Fairgreaves

So far in the series, we’ve looked at some big physics concepts, but now, we’re trying something a little different. In this post, we are going to have a look at some of the practical ways we use physics in our daily lives. Specifically, we’re looking at detectors.

Smoke detectors

Air molecules ionized by the radiation given off by americium-251 can detect smoke particles. Unfortunately, they can’t tell the difference between smoke particles caused when your house is burning down, and when you’ve put your crumpet down one too many times in the toaster | Image: Whitepaw

Hopefully, you are pretty comfortable with what a smoke detector is, and why it is useful- but have you ever wondered how they work? Surely that’s not physics!?

Well, surprisingly- yes it is.

Inside your humble smoke alarm is a small amount of americium, a radioactive material. The presence of this radioactive material means the particles of air inside the detector are ionized, allowing an electric current (more on that next week!) to flow across the chamber.

When smoke particles go into the detector some of the ionized particles attach to them. The current drops, setting off the alarm.

There are actually two different types of smoke detectors. Most common is the type that uses an electric current generated by radioactive material but there is another type.

Photoelectric detectors work in a totally different way but are still a pretty good example of how physics can help us out every day. Under normal conditions, in a photoelectric detector light travels from one end of the chamber to another unobstructed. When smoke enters the chamber the light hits the smoke particles, this causes the light to scatter and hit a sensor which then triggers the alarm to sound.

And now you know what is going on when you burn your toast!

Metal detectors

Heheh… those fools will never know about my emergency supply of plastic knitting needles! | Image: Hunter Desportes

I don’t know about you but when somebody says metal detectors I still think of somebody trawling beaches in search of buried treasure. Of course, there are also much more serious applications of this technology. Those beeping things at airports? They are metal detectors too, and they work in more or less the same way.

Metal detectors rely on the fact that an electric current will always generate a magnetic field in a direction perpendicular to its movement. And likewise, a magnetic field across a wire will always induce a current in the wire.

When forming wires into coils of many loops, strong magnetic fields can form through the loop of the coil, simply by firing a current through the coil. Metal detectors can cleverly use these physical laws to their advantage.

In an airport, metal detectors work by firing currents which rapidly alternate in direction through coils of wire – producing a magnetic field which itself alternates in direction. The presence of metal in your pockets, belt or jewellery as you pass through can be enough to disrupt this magnetic field.

Side-view of a magnetic field as it compellingly comes through a current-carrying coil in characteristic curves | Image: Geek3

Due to an important affect in physics known as Lenz’s law, small currents are induced in the metal on your body, which produce their own magnetic fields to resist the field of the detector.

Another coil of wire in the detector is there to measure the overall magnetic field. This time, instead of having an alternating current fired through it directly, a current will be induced by the magnetic field – the strength of the field can be measured by measuring the strength of the induced current.

If the overall magnetic field is any different to that created by the initial coils, the detector will know something is up. An opposing magnetic field was generated as you passed through, which can only mean that there is metal on your body, and the detector sounds the alarm.

Radiation detectors

Geiger and Muller had some trouble persuading everyone that their invention wasn’t just a silly looking hammer, and could actually save thousands of lives | Image: Mrjohncummings

Well, this one might seem a little more abstract but you’d be surprised to realise how common radiation detectors are.

Radiation detectors can be given a number of names, but a Geiger counter or Geiger-Muller tube is a pretty common choice. “Geiger” and “Muller” refers to the scientists Johannes “Hans” Wilhelm Geiger and Walther Muller who discovered an improved method of detecting radiation way back in 1928.

The Geiger-Muller tube detects ionizing radiation such as alpha particles, beta particles, and gamma rays. Once radiation has been detected an electric pulse is produced which shows on the Geiger counter display allowing you to get out sharpish if things start to reach scary levels.

No, it’s never a good idea to play a game of ‘Who can make the Geiger counter beep the most?’| Image: Cygaretka

Sounds interesting but not exactly relevant? Think again.


Radiation detectors can be found all over the UK in schools, hospitals, airports and of course power plants. Any workplace that handles, or could handle, radioactive material must use one of these clever little detectors to ensure that radiation stays at a safe level.


Well, radiation can be pretty dangerous stuff if you’re exposed to it for too long. With a range of effects ultimately all stemming from corrupted cell functions it’s really important to stay safe. You may remember the suspicious poisoning of Alexander Litvinenko? Exposure to radiation can be deadly.

Until next time!


  1. Every metal detector can detect any metal objects very easily. Most important part of a metal detector is metal detector coil. If your coil will be destroyed then your device not working properly. So, you should care your metal detector. I think metal detector is useful things for every losing items person.


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