The Gallilean Thermometer

"Forty below tonight -- better plug the car in."

"Thirty-six above? In January? I hope they've sanded the roads!"

The expression of temperature in numbers seems natural today. But the idea of assigning numbers to temperatures is only a few centuries old.

Early thermometers, along with most of those in home use today, were based on the same principle that assists in opening jar lids held under hot water. Most substances expand when heated, but the expansion differs for different materials. Glass expands only slightly when heated, but metal jar caps, mercury and alcohol expand much more for the same increase in temperature.

The difference would be hard to see if you were looking at a simple glass tube of alcohol, so our familiar alcohol-in-glass thermometers magnify the expansion by putting alcohol (or another fluid that expands uniformly with any increase in temperature) in a bulb with only a thin tube available for the fluid to expand. The increase in volume of all of the fluid in the bulb has to be taken up in the thin channel of the thermometer stem, so a small change in temperature can be seen as a relatively large change in the position of the fluid in the tube.

Although the bulb and stem thermometer is the common type today, it is not the only way to use the change of volume of a liquid to measure temperature, nor was it the first way. Gallileo invented a thermometer some 350 years ago that was based on measuring the change of density of a liquid, rather than the change of volume directly. Density and volume are closely related, since the density is just the mass of the liquid divided by its volume, and the mass does not change as it is heated. Measuring density is quite easy if you have a number of objects of known density for comparison -- if an object floats, the density of the liquid is greater than that of the object; if it sinks, the object is denser than the liquid.

If you are primarily interested in measuring temperature, you don't even have to know the densities of the objects -- you just keep a record of what floats and what doesn't at different temperatures. If you don't have a thermometer for comparison, you can calibrate your objects in terms of ice formation, body temperature, or other non-numerical measures of heat or cold. By using objects that vary widely in density, you can get a thermometer that has a very wide temperature range; by using very similar objects, you can make a thermometer which is extremely sensitive. You can even combine the two, using several objects with a wide spacing of density to get a coarse thermometer with a wide range plus several more objects very close together in density to measure a particular temperature range very accurately. The result could be a thermometer with the accuracy of a fever thermometer in a narrow range of temperatures, but capable of measuring approximate temperatures from forty below to over a hundred degrees F.

Gallilean thermometers are available today, though they are sold more as art objects than as instruments to measure temperature. The ones I have seen are calibrated to a rather narrow range of typical indoor temperatures, with the floats made of glass with varying amounts of colored liquids and air enclosed. The float density is fine tuned by the addition of small metal weights. Different floats vary by about one degree F in the temperature at which they change from sinking to floating. The lower the temperature, the denser the liquid becomes and the more floats will rise to the top of the cylinder. Individual floats are labeled with the temperature at which the fluid density just equals that of the float.

Gallilean thermometers do have drawbacks. The large volume of liquid required to contain the floats warms and cools slowly, so the temperature as recorded by the thermometer may not change nearly as fast as the temperature of the surroundings. And they are not very portable -- a Gallilean thermometer would hardly adapt to being a zipper pull! But they provide an attractive way of monitoring your living room temperature.