To go back to the three bowls of water, what we are looking for is some operational definition of the ‘hotness’ of the water that doesn’t depend on how it ‘feels’.  That is, you could put a rock in the water and then see how hot the rock becomes, but that would still be a matter of how it felt.  What you want is something that changes with things that are hotter.  The oldest device in this vein that I know of was developed by Galileo, and it turns out to be really easy to reproduce.  All you need is a simple set of glassware and some water.

A typical set-up uses a flask at the top with a one-hole stopper and a length of glass tubing running down to a beaker of water.  I would usually put out a range of options: Erlenmeyer and Florence flasks of different sizes, different lengths and diameters of tubing, and different sizes for the beaker at the base.  A former student who did this lab with his middle-school classes devised a delightful inexpensive and low-tech variation that is just fine.  It uses a small glass bottle, like a ‘Veryfine’ juice bottle.  Drill a hole through the plastic cap large enough for a transparent straw, and then use glue to seal the fit of the straw.  A plastic cup works just fine as a beaker.

The next step transforms the glassware into a thermometer.  First put water into the beaker so that the tubing (or straw) is standing in water, though not touching the bottom of the container.  By the way, you should put some food coloring into the water to make it easier to see.  We want the water to stand about midway up the glass tubing or straw.  Ask students how they might get this to happen.  They might come up with a good idea or two.  If not, ask what would happen if you heated the air in the flask or bottle at the top.  You might try it with a set-up of your own.  The air expands as it’s heated and pushes its way out the only opening…the bottom of the glass tube or straw.  If we used a candle to heat the flask and then pull it away, the air can now cool.  What happens now?  The air will contract and the water will rise up the tube.

This is a great time to talk about vacuums, and the difference between seeing a vacuum as sucking things up and surrounding air pressure as pushing things in.  I would usually offer the plausible maxim that “nothing is not an agent”.  If there is nothing to a vacuum, how could the nothingness actually pull anything?  This conversation would itself lead to all sorts of notions and side explorations with a vacuum pump.

But, let’s get back to the thermometer.  If you have guessed how much air escaped by counting bubbles, for example, and it was a good guess, the water will stop near the middle of the tube, and now you have a thermometer!  That is, you have a device that is sensitive to changes in how hot something is, but at which end?

But let’s leave this for next week…