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How Do Smoke Alarms Work?

The smoke alarms in my apartment building are both ridiculously sensitive and ridiculously loud. They regularly go off even when there’s no smoke, and I often have to scramble up on top of a chair to reset them because a pot of boiling water is producing steam. If I stand in the hallway for a few minutes just before dinner time, I can hear them go off in the different apartments, followed by my neighbors’ strings of profanity and rushed footsteps. This got me wondering: How do these things work?

Where There's Smoke ...

There are two main types of smoke alarms used in homes: ionization detectors and photoelectric detectors. Inside the ionization ones there’s an ionization chamber with two plates and a source of ionizing radiation. The alarm’s battery sends a voltage to the plates, charging one positive and the other negative. The radiation source, a small amount (around 1/5000th of a gram) of an isotope called Americium-241, decays and emits alpha particles (subatomic particles made of two protons and two neutrons) at a reliable, constant rate. As the particles travel through the chamber, they ionize, or knock an electron from, the oxygen and nitrogen atoms in the air that passes through the chamber.

The newly free electrons, which have a negative charge, are attracted to the positively charged plate, and the now-positively charged atoms are attracted to the negative plate. This maintains a small but constant current between the two plates. When smoke enters the chamber, it disrupts this little dance of ionization and lowers or kills the current between the plates, triggering the alarm.

Now, if the idea of radioactive isotopes hanging from the ceilings in your home alarms you a little, and quick death by fire sounds more attractive than slow death by radiation, let me put your mind at ease. Alpha particles have very little penetrative power. They can’t get through the plastic of the detector, and if they did escape, they can’t travel very far in regular air. Because of the small amount of Americium in there, and the design of the detector, there’s no health hazard unless you monkey with the chamber and directly expose yourself to the particles (that is, inhale or ingest them).

The two big drawbacks to ionization detectors are that the radioactive isotope requires proper disposal of old detectors so that they don’t pose a hazard, and that their design is very sensitive (to detect hot, fast fires that produce very little smoke). This means, as I can attest, that they’re prone to false alarms caused by dust and steam and other vapors.

Ray of Light

The other common kind of detector, a photoelectric detector, contains a light-emitting diode that sends a beam of light across the top of a T-shaped chamber. At the base of the T is a photocell that detects light. When smoke enters the chamber, the light hits it, gets scattered into the base of the T and strikes the photocell.

When a certain amount of light hits the cell, it triggers an electrical current that sets the alarm off. These detectors are not as sensitive as the ionization ones and are designed to detect slow, smoldering, smokier fires.

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Big Questions
Why Does the Queen Have Two Birthdays?
CHRIS JACKSON, AFP/Getty Images
CHRIS JACKSON, AFP/Getty Images

On April 21, Her Majesty Queen Elizabeth II will turn 92 years old. To mark the occasion, there are usually a series of gun salutes around London: a 41 gun salute in Hyde Park, a 21 gun salute in Windsor Great Park, and a 62 gun salute at the Tower of London. For the most part, the monarch celebrates her big day privately. But on June 9, 2018, Her Majesty will parade through London as part of an opulent birthday celebration known as Trooping the Colour.

Queen Elizabeth, like many British monarchs before her, has two birthdays: the actual anniversary of the day she was born, and a separate day that is labeled her "official" birthday (usually the second Saturday in June). Why? Because April 21 is usually too cold for a proper parade.

The tradition started in 1748, with King George II, who had the misfortune of being born in chilly November. Rather than have his subjects risk catching colds, he combined his birthday celebration with the Trooping the Colour.

The parade itself had been part of British culture for almost a century by that time. At first it was strictly a military event, at which regiments displayed their flags—or "colours"—so that soldiers could familiarize themselves. But George was known as a formidable general after having led troops at the Battle of Dettingen in 1743, so the military celebration seemed a fitting occasion onto which to graft his warm-weather birthday. Edward VII, who also had a November birthday, was the first to standardize the June Trooping the Colour and launched a tradition of a monarchical review of the troops that drew crowds of onlookers.

Even now, the date of the "official" birthday varies year to year. For the first seven years of her reign, Elizabeth II held her official birthday on a Thursday but has since switched over to Saturdays. And while the date is tied to the Trooping the Colour in the UK, Commonwealth nations around the world have their own criteria, which generally involve recognizing it as a public holiday.

Australia started recognizing an official birthday back in 1788, and all the provinces (save one) observe the Queen's Birthday on the second Monday in June, with Western Australia holding its celebrations on the last Monday of September or the first Monday of October.

In Canada, the official birthday has been set to align with the actual birth date of Queen Victoria—May 24, 1819—since 1845, and as such they celebrate so-called Victoria Day on May 24 or the Monday before.

In New Zealand, it's the first Monday in June, and in the Falkland Islands the actual day of the Queen's birth is celebrated publicly.

All in all, just another reason it's great to be Queen.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
What Is the Meaning Behind "420"?
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Whether or not you’re a marijuana enthusiast, you’re probably aware that today is an unofficial holiday for those who are. April 20—4/20—is a day when pot smokers around the world come together to, well, smoke pot. Others use the day to push for legalization, holding marches and rallies.

But why the code 420? There are a lot of theories as to why that particular number was chosen, but most of them are wrong. You may have heard that 420 is police code for possession, or maybe it’s the penal code for marijuana use. Both are false. There is a California Senate Bill 420 that refers to the use of medical marijuana, but the bill was named for the code, not the other way around.

As far as anyone can tell, the phrase started with a bunch of high school students. Back in 1971, a group of kids at San Rafael High School in San Rafael, California, got in the habit of meeting at 4:20 to smoke after school. When they’d see each other in the hallways during the day, their shorthand was “420 Louis,” meaning, “Let’s meet at the Louis Pasteur statue at 4:20 to smoke.”

Somehow, the phrase caught on—and when the Grateful Dead eventually picked it up, "420" spread through the greater community like wildfire. What began as a silly code passed between classes is now a worldwide event for smokers and legalization activists everywhere—not a bad accomplishment for a bunch of high school stoners.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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