There are several ways we can make hydrometers. Why should we make hydrometers? What is the purpose of an hydrometer? Some of us don't even know how to say it. ("High-DRAW- mitter.") An hydrometer is used to tell us the specific gravity of a liquid. That doesn't tell us very much, either, does it? Mr. Webster explains it by telling us that it is kind of like weighing. It is a way of finding out the density of a liquid by comparing something we know (plain water) with something we don't know (salt water, for example). The specific gravity is the amount of weight that will float in a liquid we know and the amount of weight that will float in another liquid. It shows us the difference in density between plain water and salt water. Or between plain water and liquid soap, or syrup, or oil. Remember, it is the difference that is important.

More dense liquids float things higher, and will float heavier things. Mercury is a liquid. It is the silvery stuff we see in the fever thermometer that Mom puts in your
mouth when you don't feel well and she thinks you may have a high temperature. Anyway, even a hammer will float in mercury!

Oil is more dense than water. If you pour oil onto water, the oil floats on the water. I wonder if mercury would float on water? I don't think so, but I really don't know.

Anyway, now that we know a little bit about why we need them, we will learn to make hydrometers.

We all know that pencils are made of wood, and that wood floats in water. That means that a wooden pencil will float in water. It will just lie there, on top of the water. If
we put a thumbtack into the eraser of a pencil, it will make the eraser end heavy, and that end of the pencil should sink. (Plastic thumbtacks won't work. Use metal tacks.)

The container we have here is deeper than a pencil is long. Put the pencil, with the tack in the eraser, into the water. Wowie! Look at that! That pencil is standing up in the water, with the eraser pointing down, but it is still floating. That really looks weird, doesn't it?

Carefully, now, with a felt pen, we'll mark the water level on the pencil so we won't forget how deep the eraser went into the water before it stopped sinking. The pencil comes out of the water now, and three tablespoons of salt will be dissolved in it. Salt dissolves fast if it is put into a small amount of hot water and stirred for awhile. Then, pour that into the water we are using for our experiment.

Now, we'll see how the pencil likes salt water. [If we want to talk like geologists, we can call it saline ("SAY-leen") water. If we want to talk like pickle-makers, we can call it light brine.]

The pencil doesn't like it much, does it? It doesn't want to go all the way in! Look at the water line where we marked it before adding salt to the water. Now, the water line is closer to the eraser, because the pencil is floating higher. It is floating higher because the saline water is now more dense than plain water. The difference in the depth to which the eraser sank in plain water and the depth to which it sank in the saline tells us the density of the saline.

We know that even very heavy oil floats on salt water, because that is what spills onto the ocean water when an oil tanker has an accident. We also know that oil is dense, because we know that it keeps oxygen from going into the water below. That's one of the way that sea animals and plants die when there is an oil spill.

Let's add three more spoons of salt and see if the pencil floats even higher.

Now, let's rinse the pencil and dry it, making sure the waterline we marked when it was in plain water stays on it, and float it in some cooking oil. Is the cooking oil more
dense or less dense than the saline? I wonder if the new "light" cooking oil means that it is less dense than regular cooking oil? We can find out with our hydrometer.

There are other ways to make this instrument. We can seal one end of a plastic drinking straw with melted wax, and pour in a little sand. We want to go easy on the sand, because too much would make the whole thing sink. Probably if we hold the straw in the water while we add some sand, we can see when the straw stands up straight like the pencil did. A bit of glue will keep the sand in the end, where it belongs. If we get it right, the straw should work just like the pencil.

We need to know how to use this information. We need to name the amount of specific gravity of each different liquid. To do this, the waterline mark on the pencil or the straw can be called "one unit" and the waterline when we floated them in the heavy saline can be "ten units." Maybe the waterline from floating them in the light brine is "five units." We'll have to check and see.