Traditional thermometers contained mercury, which expands with rising temperatures. But most households have digital thermometers now because they’re safer, easier to read, and work faster. Digital thermometers contain an electric resistor, also known as a thermistor, which is temperature-sensitive. When the temperature rises, the thermistor becomes more conductive. This happens at about 37°C (99°F). A microcomputer pinpoints the temperature by measuring the conductivity, and displays it on an LCD screen.
Originally, Anders Celsius pegged his scale with the boiling point of water at 100 degrees and the freezing point of water at 0 degrees, based on the water’s behaviour under pressure, but Carl Linnaeus swapped these after his death. Daniel Gabriel Fahrenheit ﬁrst based his scale on three states of brine: stable, freezing and boiling. Later his scale was adjusted so there were 180 intervals between the freezing point of water (32°F) and boiling point of water (212°F). The scales intersect at -40 degrees.