Although various types of thermometer are available today, the principle that led to the development of the first type – that some substances expand and contract with a change in temperature – has been known for thousands of years and is noted by some Greek philosophers.
This knowledge led to the development of the thermoscope, which is basically a liquid thermometer without the scale. Scientists in the sixteenth and seventeenth centuries, most notably Galileo Galilei, developed similar thermoscope devices.
Between 1709 and 1724, Daniel Gabriel Fahrenheit created the first recognised thermometer with a scale. Fahrenheit first invented the alcohol thermometer, then the mercury thermometer, and finally the fixed scale which is named after him.
The Fahrenheit scale was originally set with zero being the temperature of an equal-parts mixture of water, ice and salt and one hundred degrees as human body temperature. With scientific advancements and improved accuracy, it was discovered that body temperature is actually 98.6 degrees Fahrenheit.
Anders Celsius invented the centigrade scale in 1742. The word centigrade refers to the division of a range of measurement into one hundred parts. Celsius divided the temperature range from the freezing point to the boiling point of pure water into one hundred degrees. Celsius was adopted as the name of the unit in 1948.
Lord Kelvin invented his scale in 1848. This incorporates the lowest temperature possible, at which air will freeze: absolute zero (-460°F, or -273°C). The kelvin scale is generally used in more professional applications and not found on digital thermometers.
From iconic liquid type thermometers and through scientific discoveries, other types of thermometer were developed, including the digital type. To learn how a digital thermometer works, see: How does a digital thermometer work?