675. How do these digital thermometers work? I read somewhere that they contain alcohol instead of mercury. — KM, Lincoln, NE
Most modern liquid-in-glass thermometers do contain alcohol rather than mercury, but these aren't the digital thermometers you are referring to. The alcohol thermometers are the ones with the red line that moves upward in a glass tube as the temperature increases. I believe that the digital thermometers you're interested in are the ones with numbers that change colors as the temperature changes. For example, when its 72° F, the number "72" is brightly colored while the other numbers are essentially black. Those thermometers use liquid crystals to measure temperature. More specifically, they use chiral nematic liquid crystals—long asymmetric molecules that arrange themselves in orderly spirals in the liquid. When light strikes these spiral structures, some of it reflects. But the reflection is strongest when the light's wavelength is an integer or half integer multiple of the spiral's pitch—the distance between adjacent turns of the spiral. Since light's wavelength is related to its color, the light reflected by these liquid crystals is colored. Because the pitch of a chiral nematic liquid crystal changes with temperature, so does its color. Slightly different liquid crystals are inserted behind each number on the thermometer so that each number becomes colored at a different temperature.