- [Jared] Most scales
nowadays are digital, they use electricity to determine
how much something weighs. What I'm interested in is these
old style mechanical scales, sometimes called spring scales. No batteries required here. In this video, we'll take
a part two of these scales to see how they work on the inside. (upbeat music) The idea of using springs in a scale has been around for about 200 years now. This one is called a floor scale and this one is called a hanging scale. So let's spring right
into the details here, and we'll start with the hanging scale. These are commonly used
at the grocery store. The more weight or the harder
that you pull on the bottom, the more that the red
pointer spins on the inside. Take away the weight and the
red pointer goes back to zero. Let's take it apart and
see what's on the inside. The first thing you'll notice are two large springs on the inside. They're hooked onto the top piece, which is bolted to the frame of the scale. In the center, we have the gear system. This piece is called the rack. Hidden inside of here is
called the pinion gear. As the rack goes back and forth,
the pinion gear spins too. The rack is bolted to the
frame that the springs are on. So when the springs go down, the rack goes down too,
which then spins the gear. There's another tiny spring
here which pulls on the rack. This ensures that we have good
contact with the pinion gear. The center of the gear goes out and pokes through the center of the dial so that we can attach the red pointer. Now, as the scale is pulled
down, the spring stretch, which moves the rack, turns the
gear, turns the red pointer, which then shows how
much the object weighs. Now, may happen that the red
pointer starts to get off. It's not pointing directly at zero when there's nothing on the scale. This might happen when you
attach a basket to the scale. In this case, we need
to calibrate the scale. That's the job of the calibration knob, also known as a zeroing screw. You can see that it goes all the way through and
comes out the other side. These two are called the pivot plates. They both have a hole in the middle which is where they attach. As you spin the calibration knob, it will push against
the metal pivot plates. So as you turn the knob, the whole spring assembly
will go up or down, which will just barely
move the red pointer. Once it's right at zero, then you are ready to
start weighing again. The reason that a scale like this works is because of something
called Hooke's Law. It's the relationship
between how much force you pull on a spring versus
how far that spring stretches. So just for an example, let's put a one kilogram
object on this spring. It stretches one centimeter. Okay, now let's double the weight. Now it stretches two centimeters. For each extra kilogram, the spring stretches another centimeter. If you put it on a graph, then it's a straight line, or in other words, it's
a linear relationship. This means it's predictable. We can now use the spring to determine the weight of an object, figure out how far the spring stretches, which will then tell us
the weight of the object. For this spring, the numbers are easy. One kilogram for every one centimeter. But maybe you've got
a really stiff spring. This one takes five kilograms to stretch it only one centimeter. Maybe you've got a really flexible spring. It takes barely any weight at all to stretch it one centimeter. When you study Hooke's
Law in your physics class, you might see an equation
like this, F equals kx. The K is how stiff the spring is, x is how far the spring is stretched, which results in a downward force or how heavy the object must be. Hooke's Law only works
up to a certain point. Normally, you take the
weight off the spring, and the spring goes back
to where it started. However, if you put a very
large weight on a small spring, you will probably stretch
that spring so much that it won't go back. You've reached the elastic limit, and Hooke's Law no longer works. This spring is somewhat
useless to use in a scale now. So keep that in mind, it is possible to break these scales. With the hanging scale, there's two springs in
here working together. Since we know how stiff they are, we can use how far they
stretch to turn a gear, which then tells us how
much an object weighs. Now let's take a look at the floor scale. Two tiny springs, hold it all together. It's easiest to unhook it from the bottom. The front cover comes off and
the insides become visible. The big difference here is that instead of a red
pointer moving across the dial, this time the whole dial actually moves while the red pointer is stationary. The dial is attached
to another pinion gear with a rack moving across it. This of course is very similar to the hanging scale that we saw earlier. The end of the rack is
attached to a small spring which pulls it towards the edge. Things get interesting at
the other end of the rack. We've got a lever that
can pivot back and forth. It's attached to the rack, and remember, the rack is
constantly being pulled this way. The only thing stopping it is another metal plate right beneath it. This plate is held up by our main spring. There's only one of them this time, and it's quite a bit smaller. Our spring is still hanging but the end of it is
attached to a metal plate. When the plate goes down,
the spring is stretched. This also means the lever
is allowed to rotate. The metal plate is pressed down by four metal bars that
go across the scale, two long ones and two shorter ones that hang right beneath it. All four of the bars also rest on the edges of the scale case. The lid to the scale has
four supports on the bottom. Each of these supports rests on one of the
four bars in the scale. When you stand on the scale, your weight is distributed
down through the four bars, over to the metal plate, which moves down and stretches the spring, allows the lever to rotate,
which moves the rack, which then rotates the
gear that moves the dial. No matter where you stand on the scale, your weight gets distributed
to the tiny spring which uses Hooke's Law
to determine your weight. Just like with the hanging scale, this one may need to be
zeroed out if it gets off. The calibration dial is down here. There's two parts to this,
the bottom which can spin, and the top, which fits inside of it. Notice the screw threads around the side. When you spin the bottom,
the top goes up or down. This moves the resting
position of this spring, which will then ripple through and affect where the dial is when there's
no weight on the scale. My name's Jared. I make 3D animations on how things work. If you enjoyed this video, hit that subscribe button and the bell so you're notified when
I make a new video. Thanks for watching, and
I will see you next time. (upbeat music)
