Baby Thermometers

Babies have very limited methods of communicating what they want or what is wrong with them. Crying can mean any number of different things. At this point, I am fairly good at figuring out when my daughter is tired or hungry or upset about strangers or just bored. However, sometimes she just cries and gets very upset for no clear reason and it is difficult to figure out what is wrong.

A couple of weeks ago, my daughter was very fussy and needier than usual and just refused to nap. After a while, I decided that there was definitely something wrong and that she seemed sick. One of the main tools for determining if our child is sick is our thermometer. Being a scientist (or a geek, if you prefer), I am of course interested in how different thermometers work and how they measure my daughter’s temperature.

While it seems that there are a wide variety of baby thermometers – ear thermometers, rectal thermometers, under arm thermometers and forehead thermometers – the most common types fall into just two categories of operation. The rectal and under arm thermometers use contact between the skin and a metal component on the thermometer to read temperature while the ear and forehead thermometers use the radiation emitted by your baby’s body to calculate temperature. These thermometers use very different techniques to measure temperature than the bulb thermometer many of us grew up with.

Contact Sensors

The metal contact sensors are most commonly electrical sensors. There is a thermistor in the metal detector portion of the thermometer. This is a resistor that changes its resistance depending on temperature. To understand how this works, we need to think about some simple electronics.

Consider a very simple electronic circuit like the one below:

A battery provides a voltage – 9 Volts in this picture. A constant current travels around the circuit because there is just a single path through the loop. An ammeter is a device that is used to measure the current – this is a measurement of how much charge passes through that point in a given amount of time.

Ohm’s Law tells us that voltage, V, is related to current, I, and resistance, R, as follows:

So what happens when my resistor is sensitive to temperature? When you put the resistor in contact with your body, it heats up and its resistance decreases. The battery voltage stays constant, so as the resistance decreases, the current through the system must increase. This makes sense, right? If there is less resistance to the flow of charge, more charges will pass the ammeter in a given amount of time.

By measuring this current, and knowing the voltage of the battery, you can easily calculate the resistance of your temperature sensitive resistor using the equation above. The resistance vs. temperature is something that is very well known by the manufacturer, so once you know the resistance, you know the temperature of your baby who is in contact with the thermometer.

The circuit in your thermometer has a few more components, but this basically how it works. And it does all the calculating for you and just displays a temperature.

Infrared Sensors

If you bring your child to the doctor’s office, they will likely take his or her temperature using an infrared sensor. This is what my nurse uses. She takes a device with a flat, round end and lightly moves it across my daughter’s forehead and reads out the temperature. This is very fast and is great for a kid (like mine) who hates to be touched by strangers.

How does this work? If you remember, a few weeks ago, we talked about Blackbody Radiation. Our bodies emit light, but it is in the infrared where we cannot see it with our eyes. The peak wavelength emitted from our bodies is about 9 microns (Check out the PhET website to see for yourself). The amount of light and the peak wavelength of the light both depend on the temperature of our bodies.

The thermometer uses a thermopile to measure the radiation coming from our bodies (How Stuff Works). Heat from our bodies heats two pieces of metal that make up the thermopile. A voltage is created between the pieces of metal that depends on the heat of our bodies. (Wikipedia)

How do we measure this voltage and turn it into a temperature? We could use the same circuit that we used above. This time, have a constant resistor and use the thermopile as our voltage source. We can measure the current to determine the voltage created by the heat from our bodies and determine the temperature from that voltage.

The infrared sensors are very fast, which is definitely a plus with a screaming, squirmy baby. Both types of thermometers work best when the temperature is taken inside the body – a rectal thermometer or ear thermometer. However, we were discouraged by the hospital from using a rectal thermometer on a newborn because they said it is to easy to cause them harm because they are so tiny. And ear thermometers can be a little tricky because they need to be aimed at the baby’s eardrum to get an accurate reading.

We have one of the forehead thermometers at home so I definitely wanted to make sure I understand how they worked. It turns out my daughter did not have a fever when she was sick a few weeks ago. I did take her to the doctor, though, and she did have an ear infection. The poor kid was sick for a while since the first set of antibiotics did not work on her, but is feeling better. And is now fussy for some other completely unknown reason…

I hope my daughter continues to be healthy and not have any fevers, but I do think it is interesting to know how current thermometers make use of technology to help me figure out what is going on with my little girl.

And, of course, I am looking forward to the time when she can tell me what is wrong and there is a lot less guesswork in trying to figure out if she is sick.


Solar eclipse

There is going to be a solar eclipse visible in the Western United states this evening (and in east Asia, but I live in the US). This will be the first solar eclipse for my daughter, so hopefully the weather cooperates and we can go out and see it. Of course, she probably won’t even notice it and I should not encourage her to look at the sun anyway.

What is a solar eclipse and what causes it?

We know that the Earth orbits the sun and the moon orbits the Earth. Sometimes these orbits are lined up so that the moon passes between the Earth and the sun, blocking the light of the sun from reaching us. This causes a solar eclipse.  Below is a picture from Wikipedia Commons illustrating this effect.

 The umbra is the area on the Earth where the light from the sun is completely blocked. This is a total eclipse. The penumbra is the area where the light from the sun is only partially blocked. Outside these areas, an eclipse will not be viewed – the moon will not block the light from the sun at all. 

The eclipse this weekend is going to be annular eclipse for some areas. This is what you see when the moon passes directly in front of the sun, but is not large enough to block the entire sun. Some light makes it past the edges of the moon and is seen on Earth.

Below are some rough drawings of what the different types of eclipses look like:

The type of eclipse depends on the exact path of the moon and where on Earth you are viewing the eclipse.

Why does the moon sometimes block all of the light from the sun and sometimes only some of the light? The amount of light blocked depends on the moon’s distance from the Earth. When the moon is closer to the Earth, it casts a bigger shadow, and when it is far from the Earth, it casts a smaller shadow. To understand this, close one eye and hold your thumb up in front of the other eye. Now look at something far away.  If you hold your thumb close to your eye, it likely blocks the entire object you are looking at. However, if you move your thumb as far away from your eye as you can, it blocks a much smaller area of the object.

The moon does not orbit the Earth in a perfect circle. The orbit is an ellipse (oval shaped). That means that sometimes the moon is close to the Earth and sometimes it is farther away.  The closest point is called the perigee of the orbit and the farthest point is the apogee of the orbit. When there is a full moon at the perigee (or close to it), it is sometimes called a ‘supermoon’ because the moon seems very large and bright. This happened earlier this month, on May 5 (the perigee was on May 6). A calendar of apogee and perigee dates can be found here.

Eclipses only occur during a new moon. The new moon is when the moon is dark – the opposite of the full moon. A full moon occurs when the the light side of the moon is facing the Earth. This is caused by the light of the sun reflecting off of the moon and hitting the Earth where we see it since the moon does not emit visible light on its own. If the moon is blocking the sun, as during an eclipse, the light side must be facing the sun (and the light reflects away from us)and the dark side must be facing the Earth.

Today, there is a new moon close to the lunar apogee (May 19), so the moon will be almost as far away as it gets. Therefore, the shadow it casts will not completely block out the sun, allowing for some people on Earth to see an annular eclipse.

Will I get to see the eclipse?

Solar eclipses are fairly common on Earth, but each eclipse can only be seen by a relatively small area of the Earth. Therefore, eclipses at a particular location are not that common. If you live in the Western US, there will be an annular eclipse today, May 20, 2012. Much of the Western US will only see a partial eclipse. If you would like to find out what time the eclipse will occur at your location, or when the next solar eclipse will be, check out NASA’s Solar Eclipse page.  This page requires you to either pick the city closest to you, or enter the exact coordinates of your location. If you would like to enter your coordinates, you can find most towns and cities on Wikipedia, with the coordinates listed on the right hand side of the page. Or you can use Google Earth to find the exact coordinates of your house. NASA also has a special page with information on today’s solar eclipse.

We will not be in the area where the annular solar eclipse will be visible, but I am excited about being able to see the partial solar eclipse tonight.

Babies in motion

It is amazing how much of introductory Physics we all learn before we turn one year old. We learn it so well and internalize the information that we forget that we even know it. It is just a part of our core knowledge, like breathing and eating (other things we learn as babies).

My daughter is just learning these things for the first time. She is just beginning to explore motion. Here are some of the things that she has learned so far:

  • The smooth floor at mommy’s yoga class is good for sliding on. You can scoot while sitting up to get closer to the good toys (like yoga blocks and straps).
  • The rug is best for rolling over and standing up and sitting up.
  • The counter is not good for standing up on.
  • Sitting can be accomplished by either pulling on mommy’s hands, or both pushing on the floor and pulling on something.
  • Rolling is easiest if you first push up on your arms, or push your butt up into the air, or kick your legs up into the air.
  • Small things move when you pull on them. Parents and furniture do not.

Friction and Forces

So, my daughter has learned all about forces & friction: The smooth floor at the yoga studio has very small coefficients of both static and kinetic friction – that means it is easy to get things moving and keep them moving. In order to move, she needs to apply a force that is larger than the force of friction in order to accelerate from her current position.

One of the first things we learn in Physics is that the sum of all the forces on an object  is equal to the mass of the object (mdaughter) times its acceleration (adaughter). My daughter needs to scoot with a force greater than that of friction, and then her movement will be governed by the following:

The harder she pushes off (larger Fdaughter­), the faster she accelerates. Or if the force of friction is smaller, it’s easier to move. That’s why she can scoot on the smooth floor but not on the rug yet. Her small mass also makes it easier. I have to push much harder to scoot on the floor than she does, but then again, I’m a bit stronger as well.

Okay, so sliding is easiest on surfaces with less friction – smooth floors, countertops, icy surfaces, etc. But sometimes sliding does not help her in her attempts to move.

When she is trying to stand, she needs her feet to stay still (not slide) while she pulls up. When she’s sitting on the counter and she pulls on my hands, she just slides instead of standing up. So, for standing, she needs a big friction force.

Newton’s Third Law

She has just discovered that pulling up on my hands seems to be similar to pushing down on the floor. Why? Newton (he was a smart guy) said that every action has an equal and opposite reaction. When she pulls down on my hands, my hands (without me moving) pull up on her. That helps her sit up. When she pushes down on the floor, the floor pushes up on her, also helping her to sit up.

Of course, if she pulls on smaller objects or objects that are on a smooth surface, sometimes they move! That’s just friction again – the force of friction is directly related to the weight of the object. Heavier objects experience have a bigger friction force keeping them immobile. Lighter objects experience less friction and so it takes a smaller force to get them in motion.

It is a common misconception that she can pull lighter objects because she pulls on them harder than they pull back. That’s never true. The forces are always equal and opposite. When my daughter pulls an object to her, it is because the force of her pull is greater than the force of friction (or other force) holding the object in place.


My daughter has been rolling over for quite some time, but she’s always hated to be on her belly, so she refused to roll onto her belly. And she was so unhappy when on her belly that she just cried instead of thinking to roll onto her back.

In the past couple of weeks, though, she has remembered that she can roll over. She stays on her belly exactly as long as she wants and then rolls over. She’s also realized that she can continuously roll over and over again to move across the room. (Crawling on her belly has occurred to her but she has not yet discovered how to make that work).

We know that we need a big force to move in a line. My daughter needs to learn to push or pull (or kick) off with more force in order to scoot along or crawl. What about rolling? Is it just a matter of force? How do we rotate?

Think of opening a door. When you push a door open, it matters both how hard you push the door and where you apply that force. If you try to open a door by pushing on it very close to the hinge, you have to push very hard to get it to open. But if you push on the edge of the door furthest from the hinge, you do not have to push very hard at all. So rotating objects seems to depend on how hard you push (or pull) but also where you apply that force – how far from the point of rotation.

When my daughter lies flat on the ground and tries to roll over, she doesn’t go anywhere. She would have to have very strong abdominal muscles to make her body rotate – the force of her abs is applied very close to her center of rotation.

But she’s too smart for that. She’s already learned about torque (force times distance from axis of rotation) and gravity. So, when on her back, she kicks her legs up in the air (and sometimes her arms too), rotates just a little bit and uses gravity to make her legs and arms fall to the side and bring her around all the way. When on her belly, she can push up on her arms first, then just push off a little on one arm and her body falls to one side and rolls back over (kicking your butt up in the air and kicking off works just as well). She has already learned that she needs a lever arm to rotate. What a smart kid. 

Physics in Motion

My daughter has not yet mastered the art of motion, but she learns more about it every day. She is quickly mastering the Physics involved and will soon be ready to study energy, power, fluid motion, electricity and magnetism. By the time she’s five, I imagine she will be doing her graduate work in Physics. Or perhaps, she will be done with that and have moved on to learning new languages and cultures by then.

I love to watch her learn. If only we could all learn with the wide-eyed, fearless innocence of a baby. Not to mention the amazing brain capacity to learn a hundred new things every day. I guess if my brains were working at that speed all the time, I would also need two naps and eight meals a day.

Just A Mom

Sometimes I feel like just a mom.

My job is the hardest job I’ve ever had. I have to work long hours (24 hours a day sometimes). I have to be a nutritionist. A chef. A clown. A weight lifter. A doctor. A teacher. A house cleaner. A dairy cow. A mind reader. A chauffeur. A musician. An event planner & coordinator. A shopper. A soother of hurts, real and imagined. A nurse. A photographer. And the list will keep growing with her.

So how can I ever feel like just a mom?

Before my daughter was born, I had a full time job that was very intellectually challenging. I liked to be outdoors, go rock climbing, knit small projects, bake cookies, have coffee with friends, go out and drink beer, eat at interesting restaurants, go on dates with my husband, play computer games, and travel around the world.

My identity pre- and post- baby are not separate, of course. My daughter and I already spend time outdoors together and will start hikes soon. Maybe rock climbing next? I should probably wait a few years. I am looking forward to taking my daughter on travels both near and far. My love of knowledge and stories led me to read to her (including some Physics news) from before she could even sit in my lap.

Recently, I’ve tapped into my forgotten musician in my daughter’s music class. While many of the moms smile and mumble or sing quietly, I belt out the songs in my rough, off key voice. My mother played the organ at church and was always so upset that the congregation wouldn’t sing, so you can be sure her children sang every song loudly. Maybe my daughter will learn the joy of singing (in tune or out of tune) from me.

And while I don’t teach my daughter a lot of Physics yet (she is figuring gravity out all on her own), there’s no question that I’m teaching her every moment of the day.

But I still need some space that is mine. People who don’t call me ‘Mommy.’ A part of my brain and a part of my day that is devoted to that person that I used to be. It is those parts of my life that help me to appreciate my daughter’s smiles and giggles, and even her ear piercing squeals, all the more. If I do not have a break, her big wide eyed look when I come in a room seems needy. If I have some time to myself every now and then, I recognize that she’s looking at me like I am the center of her universe and that big smile means that she is happy to see me.

We all need balance, though we each find it in very different ways. I am working on finding mine.