The Science of Smoke Detectors

Many in the fire prevention industry would argue that smoke detectors are the single most important tool when it comes to preventing injury and loss of life in a residential fire. But how does this life-saving device, actually work?  How can this plastic disk detect the presence of smoke and sound an alarm? First, let’s go back in history and trace the steps back to the first smoke detectors.

American inventor Francis Robbins Upton gets the nod as the man to first invent what we all know as a smoke detector. Upton, born in Peabody Massachusetts in 1852, is most famous for being one of Thomas Edison’s main partners. They collaborated on key inventions such as the incandescent lamp and the parallel circuit grid. While working on these projects with Edison, Upton managed to create a sensor with an alarm that would sound if the temperature of a room climbed  at a rapid pace. Upton would later name this device a “smoke detector.”

In 1930, Swiss physicist Walter Jaeger would improve upon Upton’s invention when he was looking to create a device that could detect poisonous gas. His initial work did not go as planned, as the Swiss physicist was originally working toward the sensor picking up the gas and ionizing the air. As Jaeger pondered over his work, story has it that he lit a cigarette and the smoke from the cigarette caused the meter current in the sensor to drop. This discovery laid the foundation for modern smoke detectors.  

While this breakthrough occurred in the 1930’s, it wasn’t until 1965 when smoke detectors became produced for the masses. Duane Pearsall is credited with revolutionizing the fire prevention industry by mass-producing battery powered smoke detectors that could be purchased by the public. Production and manufacturing costs dropped and the fire prevention industry forever changed.

Most smoke detectors generally consist of two parts: a sensor to actually sense the presence of smoke and an alarm or electronic horn to notify someone in the case of smoke detectionWhile the components of all smoke detectors are generally the same, there are two different types of smoke detectors commonly used today: ionization and photoelectric smoke detectors.

Photoelectric detectors depend on light beams – or actually the lack of light – to detect smoke in a room. The theory is that when smoke levels rise high enough to block these beams of light within a smoke detector, the sensors trigger the siren within the detector. While good in theory, the practical application works slightly different. As you could imagine, there would need to be a significant amount of thick smoke to trigger an alarm.  This is why adjustments were needed to photoelectric smoke detectors.

The light from the beam source within the left side of a photoelectric detector shoots its beam straight across and actually misses the sensor. When the smoke enters the detector’s chamber, it is the smoke particles that scatter the beam and a small amount of the beam then hits the sensor. This is what sets off the alarm within the smoke detector.

The second type of smoke detector, ionization detectors, are  actually the most affordable (and most popular) residential detectors. Although they are an inexpensive choice, there is plenty of science going on within the detector.  Ionization smoke detectors use an ionization chamber and a tiny amount of americium,a radioactive nuclear material,to detect smoke in the air. Within the chamber are two plates with a small current between them. Within this current is a trace amount of ionizing radiation.

The particles created by the americium ionizes the oxygen and nitrogen atoms of the air within the chamber. Chemistry has taught us that “ionization” occurs when an electrically neutral atom, molecule, or radical loses or gains one or more electrons and then becomes an ion. In this case, a negatively charged electron is attracted to the plate with a positive charge, and the positively charged atom is attracted to the negatively charged plate within the detector. A tiny amount of current is produced by this “connection” and the electronics within the smoke detector can detect this.

When smoke enters the detector’s chamber, the current is disrupted when the smoke particles attach to the ions and neutralize them. The drop in the current between the two plates is picked up and the alarm is sounded.  That is an amazing amount of science in such an inexpensive device!

Of course, we can’t expect the average consumer to know or understand the inner-workings of smoke detectors. The important thing is that as many people as possible have working smoke detectors in their homes and remember to regularly change their batteries. However, that doesn’t make the science behind smoke detectors any less fascinating. From Francis Robbins Upton’s primitive temperature detection device of the 1800s to the modern marvels installed in most homes today, the smoke detector is one of the most important inventions the world has ever seen.

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