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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
How Do Aerial Skiers Perfect Their Jumps?
Cameron Spencer, Getty Images
Cameron Spencer, Getty Images

If you've ever watched an aerial skier in action, you know that some of the maneuvers these athletes pull off are downright jaw-dropping—and you've probably seen more than a few of these skiers land on their rear ends at some point. The jumps are incredible, but they're also so technical that one seemingly insignificant motion can drop a skier on his or her tail.

Given that the skiers can fly up to 60 feet in the air and come down on a 37-degree grade, it seems like just going out and trying a new trick would be a good way to break your neck. That's why you'll need one unexpected piece of equipment if you want to start training for aerials: a towel.

Instead of perfecting their flips and twists over the snow, aerial skiers try out their new maneuvers on ramps that launch them over huge swimming pools. The U.S. national team has facilities in Park City, Utah and Lake Placid, New York that include specially designed pools to help competitors perfect their next big moves. The pools have highly aerated patches of bubbles in their centers that decrease the surface tension to make the water a bit softer for the skiers' landings.

If you're an aspiring aerial skier, expect to get fairly wet. New skiers have to make a minimum of 200 successful jumps into water before they even get their first crack at the snow, and these jumps have to get a thumbs up from coaches in order for the skier to move on.

This sort of meticulous preparation doesn't end once you hit the big-time, either. American Ashley Caldwell, one of the most decorated athletes in the sport, is competing in her third Olympics in Pyeongchang, but failed to advance past the qualifiers on February 15, as she wasn't able to land either one of the two triple-flipping jumps she attempted. Still, it's this very sort of risk-taking that has brought her to the top of her game, and caused friction with more than one of her past coaches.

"Why win with less when you can win with more?" Caldwell said of her competition mentality. “I don’t want to go out there and show the world my easiest trick. I want to show the world my best trick, me putting everything on the line to be the best.”

You can check out some of Team USA's moves in the video below:

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
Is There Really Such Thing As 'Muscle Memory'?
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Is there really such a thing as 'muscle memory'? For example, in the sense of your fingers remembering where the keys of the keyboard are?

C Stuart Hardwick:

Yes and no. There is no literal memory in the muscles, but the thing people call “muscle memory” exists, though the name is a misnomer.

A better name might be “subconscious memory,” as the information is stored in the brain, but is most readily accessible—or only accessible—by non-conscious means.

What “non-conscious” refers to here is the brain’s enormous capacity to train up what might almost be called “subroutines,” that exist outside our conscious experience. I like the term for this that at least one researcher in the field uses: “zombie agency.”

Zombie agents are non-conscious, or sub-conscious (in the literal, not the Freudian sense) that can do essentially everything you can do except make value judgments. So, for example, you don’t consciously know how to control your muscles in order to walk —in all likelihood, you wouldn’t know where to begin—but your zombie agents do, and they’ll take you wherever you want to go, dodging curbs and puppies, and “waking you” when appropriate to decide which babies to stop and kiss.

Zombie agents can be rather startling things. When you suddenly become aware that you’ve driven halfway across town in the direction of the office instead of going to the shoe store Saturday morning, you have zombie agents to thank. You “wake” as if from slumber, and with the frightening realization that you’ve been flying down the highway at prodigious speed while your mind was on other things. You feel as if you’ve been asleep, and in a way you have—but a very funny kind of sleep in which it is only the uppermost layer of abstract reason that is disassociated from the rest of conscious experience. Your zombie agents have been driving to work, responding to traffic, adjusting the radio, noting the check engine light, all the things you think of as “you, driving the car,” except the big one: deciding where to go. That part was on automatic pilot (which is another good way to think of this).

This is at the advanced end of the spectrum. Typing your friend’s phone number using “muscle memory” is at the other, but it’s the same phenomenon.

We didn’t evolve to remember phone numbers, so we aren’t very good at it. In fact, we are so bad at it, we invent all sorts of mnemonic devices (memory aids) to help us [in] relating numbers to words or spacial memory, either of which are closer to the hunting and gathering we are evolved for. The illusion of “muscle memory” arises because we are supremely well adapted to manual manipulation and tool-making. We don’t need to invent a memory aid to help us remember what we do with our hands, we only have to practice.

So the conscious mind says “dial Tabby’s number,” and our fingers—or more correctly, the zombie agent which learned that task—do it. Similarly, after sufficient training, we can do the same thing with tasks like “play a major fifth,” "drive to work,” or “pull an Airbus A380 up for a go-around.”

It feels like muscle memory because the conscious mind—the part you experience as being you—is acting like a coach driver, steering the efforts of a team of zombie agents, all harnesses to collective action. But it isn’t muscle memory, it's just memory—though it may be stored (or at least some of it) in the deeper, motor cortex parts of the brain.

This post originally appeared on Quora. Click here to view.

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