We will find out in this section that liquid crystals
provide a completely different kind of thermometer.
It isn't too well suited for actually measuring the temperature,
but it is excellent for detecting small variations of temperature
from place to place.
You have met this kind of thermometer before.
There are commercially available thermometers that tell
the current temperature by having it display in brown or
green or blue;
There are fever sensors, that give a quick visual check
of the temperature of a baby or a sick person;
These materials were once sold as a novelty ("Mood Rings") --
the color was supposed to tell something about your state of
mind. What it actually detected was the temperature of your
finger!
One way to determine the amount of propane in a gas bottle
(for a gas grill, for example) is based on the fact that while
gas is being removed from the tank, the liquid is cooler than
the evaporated gas. Then a temperature sensing sheet can
show the temperature of the surface of the tank, revealing
the liquid level.
Some kinds of batteries have a battery-test strip. This
is actually a temperature sensor; you test the battery by
connecting it to a heating circuit. A dead battery will not
do any heating, and the test strip does not change color.