Save your life with radiation. Ionisation, alpha particles and a little precious metal can detect deadly smoke.
Every month Wired's chemist Dr John Emsley deconstructs an everyday product. He is the author of 110 research papers and 12 books, including Nature's Building Blocks, 2nd edition (OUP). johnemsley.com
Ingredients Americium oxide
Silver
Gold
Battery
Plastic case
Alarm horn
Printed circuit-board
Americium oxide A man-made element, all of its isotopes are radioactive. Americium-241 emits alpha particles and is used in smoke alarms as americium oxide - an alarm contains less than a microgram. When an alpha particle collides with oxygen or nitrogen, it grabs negative electrons, and thereby ionises them. When smoke enters, it absorbs the ions, so the electric current flowing through the ionised air drops. Detection of this drop sets off the smoke alarm.
Silver
One positively- and one negatively-charged silver plate create the detection chamber. Negative electrons flow to the positive plate.
Gold<span class="s2">
This low-reactive element is mixed with americium oxide and made into foil. Applied in a thin layer on to one of the silver plates, it ionises the air.
Battery
Typically, the nine-volt, long-life type is used. In new homes, where smoke alarms are connected to the mains supply, the battery is there just to provide backup.
Plastic case
The case is usually polyvinyl chloride (PVC), which is a tough, long-lasting, fire-resistant plastic. It also finds use in window frames, pipes and clothing.
Alarm horn
This emits a siren 90 decibels in volume, equivalent to hearing a motorbike engine. It has to be capable of waking someone in another room who is in a deep sleep.
Printed circuit-board Capacitors, diodes and resistors measure the electric current in the detection chamber. If it changes due to smoke particles, the alarm is triggered.
This article was originally published by WIRED UK
