How Do Thermometers Check Your Temperature?

0:00How do thermometers check your temperature? Hi! Welcome to Curious Kids Every Day! I'm Josh! How are you doing today? It's so good to hear from you. Hey, when was the last time you were sick? So sick that you were just lying in bed waiting to feel better. Did your mom or your dad check your temperature? Do you know what it means when a grown-up checks your temperature? It means they're seeing if you have a fever. See, your body temperature usually sits at 98.6 degrees Fahrenheit. If it's higher than that, that's a good sign that your body is fighting off some sickness.

1:02Now, we have a thermometer that lets you just point it at someone's head and check their temperature. But we've found that that kind of thermometer doesn't work quite as well. It's not as accurate. The ones that work the best are the kind that go into your tongue or into your ear. Have you ever had a thermometer in your mouth before or one stuck into your ear? Have you ever wondered how all those things work? How does a thermometer detect how warm your body is? How do thermometers know the temperature? How does that work? I was thinking about this question because I got an email from a curious kid named Aurora. Aurora wondered how do thermometers check your temperature? There are all kinds of different thermometers and we're going to talk about a lot of them today.

2:05A thermometer is basically a tool that compares your body's warmth to a scale. And then it shows you a number. But there are lots of different ways that thermometers can do this. Let's start with one of the oldest kinds, a glass thermometer. Have you ever seen a glass thermometer hanging on a wall somewhere? Inside the glass is a tiny tube with a liquid. And when the liquid gets warmer, the liquid expands, meaning it takes up a little more space inside of the glass tube. And since the tube is so skinny, that expansion pushes the liquid higher up. When the liquid cools down, it shrinks and slides low. So the lines and the numbers printed on the glass are placed so that the height of the liquid matches the temperature.

3:05Some older thermometers used mercury, that was the liquid they used inside, but many modern ones used safer liquids. If your parents are about my age, they probably grew up hearing the same thing I did. Do not break that thermometer, there is mercury inside of that thing. Glass thermometers now usually contain a colored alcohol, most often ethanol, which is dyed red or blue so that you can see it move. Like mercury, alcohol expands when it warms up and shrinks when it cools down. The reason they switched it is because mercury is a toxic metal that can turn into vapor and it is very dangerous to breathe. Now let's talk about the kind that goes under your tongue.

4:08Most modern, mouth thermometers are digital. And inside is a sensor called a thermistor. Can you say thermistor? A thermistor is really cool. It's a tiny piece of material that changes how easily electricity moves through it when it warms up or cools down. And how easily electricity moves through something, that has the name too. It's called electrical resistance. Imagine that you're running through some grass and then imagine that you're running through some mud. Which one is easier? Yes, the grass because it has less resistance. A thermistor is a little tool that measures how easy it is for electricity to flow through it. The thermometer has a little computer chip that measures the thermistor's resistance.

5:10And in most medical thermometers, the thermistor is designed so that when it gets warmer, it actually becomes easier for electricity to flow through it. That means the resistance goes down as the temperature goes up. Because the thermometer was calibrated, it already knows what resistance matches what temperature. So when it measures a certain amount of resistance, it can easily say, oh, that's 90 degrees or, oh, that's 100 degrees. Now let's talk about those forehead thermometers. The kind that you just point at your forehead and you click the button and then it tells you the number. Those use infrared. Infrared is a kind of light you can't see with your eyes. But warm objects give off infrared energy. The thermometer has a sensor that detects that infrared energy and estimates the temperature of the skin.

6:10That could be super convenient, but it has more things that can affect it. How far away you're holding it or whether the forehead is sweaty or whether there's hair in the way, all kinds of different things can affect it. That's why usually the under the tongue or the ear, those thermometers work more accurately. People have invented all kinds of different thermometers over the years and there are even more options now than there were when I was a kid. So who knows? Maybe when you're a grown-up, people will have invented even more ways of checking your temperature. And if this episode was especially interesting to you, maybe you could help invent one of those new future thermometers. That would be pretty cool. Alright, let's have a quick quiz.

7:10What's the name of the tiny sensor in many digital thermometers that changes when it warms up? It's called a thermistor. What kind of invisible light comes from warm things and can be used by forehead thermometers? It's called infrared. Let's play Truerfulse. Truerfulse, some thermometers work because liquid expands when it gets warmer. That is true. Truerfulse, infrared is a kind of sound that warm objects make. No, that's false. Truerfulse, a mouth thermometer can be more accurate because it's measuring a place closer to your inside body temperature. That is true too. Would you rather have a thermometer that is super fast but sometimes a little bit off or one that is slower but really accurate?

8:23Now as a dad, I will definitely choose the one that's a little slower but way more accurate. I think most parents would agree. What about you though? What would you pick? Alright, let's play around of what am I? What am I? I can happen when your body is fighting an infection. I mean that your temperature is higher than normal. And grownups often check for me when you feel sick. What am I? I am a fever. Well now you know how all the different thermometers work and you know which ones have a thermistor inside. You can impress a grownup after they check your temperature by saying, hmm, are you sure that thermistor inside that thing is working right? What kind of question do you think we'll ask tomorrow on curious kids every day? I can't wait to find out so until then keep asking questions and stay curious kids.

9:28And we've got a few birthdays today. Today is Dean's birthday. Dean is from Texas and Dean is turning five. Happy birthday Dean. Emma is from California and Emma is turning four. Happy birthday Emma. Mac is from Florida and Mac is turning four. Happy birthday Mac. Amari is from Maryland and Amari is turning five. Happy birthday Amari. Juniper is from Missouri and Juniper is turning seven. Happy birthday Juniper. To all the grownups listening if your curious kids want their birthdays mentioned on the show, please visit supportcuriouskids.com and join the Curious Kids Club. That's supportcuriouskids.com. See you tomorrow.