A thermometer is an instrument used to measure temperature. Most modern thermometers tend to be a hollow tube of glass marked with a scale and filled with a liquid, either mercury or alcohol. The thermometer works because there is a constant ratio between the amount the liquid expands and the increase in temperature. The standard mercury thermometer was invented by German physicist Gabriel Fahrenheit in the early 1700s. (He devised the temperature scale that bears his name.) The liquid is stored in a reservoir at the base of the thermometer and the column is marked off with an appropriate scale. Strictly speaking, the linear relationship between the temperature and expansion of these liquids does not hold true. For greater accuracy and reliability, the liquid should be replaced with gas. A gas thermometer has the column filled with gas at a low pressure and there is a weight at...