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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Do 'Close Door' Buttons in Elevators Actually Do Anything?
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When you’re running late for work, one small comfort is finding an empty elevator waiting for you at your office building. You scurry inside, and since no one else is waiting to enter, you jab the 'close door' button. The doors comply, the elevator starts moving, and you breathe a sigh of relief.

This is a familiar scenario for many, but it’s also a big fat lie. That’s because most of the door-close buttons in U.S. elevators don’t actually work. In fact, they’re programmed that way.

But before you get ready to send off a strongly worded email to your office building’s elevator manufacturer, you may want to hear why this is the case. When the Americans With Disabilities Act was first passed in 1990, certain requirements for elevators were outlined, such as the installation of raised buttons, braille signs, and audible signals.

The act ensured that someone with a disability would have enough time to get inside, stipulating that elevator doors must remain fully open for at least three seconds and thereby preventing the button from cutting that time short. Some elevator manufacturers took it one step further by deactivating the button entirely.

Since the life span of an elevator is about 25 years and the Disabilities Act has been around for 28 years, it’s safe to assume that most of the elevators in operation today do not have a functioning 'close door' button, The New York Times reports. Only firefighters are able to close elevator doors manually through the use of a key.

It's important to note that there are exceptions to this rule, though. As the New York Daily News noted, New York City elevators are required by law to have working 'close door' buttons, even though some operate on a long delay (so long, in fact, that it calls the button's usefulness into question).

However, you’re in luck if you’re taking a lift (which, of course, is British for “elevator”). 'Close door' buttons are fully functional in most elevators in the UK, according to The Telegraph. A spokesman for the Lift and Escalator Industry Association told the newspaper that not all elevators have the button, but when they’re present, they do work. Again, the time it takes for the doors to shut after pressing the button varies from lift to lift.

While U.S. elevator manufacturers have a seemingly good reason for disabling the 'close door' button, some may question the point of propagating the myth and installing a button that serves no purpose in the first place. In response, some would argue that placebo buttons serve an important psychological function in society.

"Perceived control is very important," Harvard psychologist Ellen J. Langer told The New York Times. "It diminishes stress and promotes well-being."

That’s right: By believing that you’re in control of your fate—or at least how quickly you can make it up to the sixth floor—you’re better off. It doesn’t end with elevators, either. Buttons placed at city crosswalks are often disabled, and the thermostats in many office buildings are rigged so that the temperature can’t be altered (even if the numbers appear to change).

Some might swear up and down that elevator 'close door' buttons work, but this, too, could be your brain deceiving you. As author David McRaney wrote in an essay: “If you happen to find yourself pressing a nonfunctional close-door button, and later the doors close, you’ll probably never notice because a little spurt of happiness will cascade through your brain once you see what you believe is a response to your action. Your behavior was just reinforced. You will keep pressing the button in the future.”

According to The New Yorker, these buttons are designed to alleviate some of the subconscious anxiety that comes from stepping inside a tiny box that's hoisted up some 20 or 40 or 80 floors by a cable: “Elevator design is rooted in deception—to disguise not only the bare fact of the box hanging by ropes but also the tethering of tenants to a system over which they have no command."

So now you know: Next time you’re running late to work, take comfort in the fact that those few extra seconds you would’ve saved by pressing a functioning 'close door' button aren’t worth all that much in the long run.

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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What’s the Difference Between Prison and Jail?
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Many people use the terms jail and prison interchangeably, and while both terms refer to areas where people are held, there's a substantial difference between the two methods of incarceration. Where a person who is accused of a crime is held, and for how long, is a factor in determining the difference between the two—and whether a person is held in a jail or a prison is largely determined by the severity of the crime they have committed.

A jail (or, for our British friends, a gaol) refers to a small, temporary holding facility—run by local governments and supervised by county sheriff departments—that is designed to detain recently arrested people who have committed a minor offense or misdemeanor. A person can also be held in jail for an extended period of time if the sentence for their offense is less than a year. There are currently 3163 local jail facilities in the United States.

A jail is different from the similarly temporary “lockup”—sort of like “pre-jail”—which is located in local police departments and holds offenders unable to post bail, people arrested for public drunkenness who are kept until they are sober, or, most importantly, offenders waiting to be processed into the jail system.

A prison, on the other hand, is usually a large state- or federal-run facility meant to house people convicted of a serious crime or felony, and whose sentences for those crimes surpass 365 days. A prison could also be called a “penitentiary,” among other names.

To be put in a state prison, a person must be convicted of breaking a state law. To be put in a federal prison, a person must be convicted of breaking federal law. Basic amenities in a prison are more extensive than in a jail because, obviously, an inmate is likely to spend more than a year of his or her life confined inside a prison. As of 2012, there were 4575 operating prisons in the U.S.—the most in the world. The country with the second highest number of operating prisons is Russia, which has just 1029 facilities.

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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