How does a thermometer tell the temperature?

A thermometer measures temperature through a glass tube sealed with mercury that expands or contracts as the temperature rises or falls.

The tiny size of the bulb and micro-fine size of the tube help the mercury reach the temperature of what it is measuring very rapidly.

Bulb thermometers follow the simple principle that liquids change their volumes relative to their temperature. As temperatures rise, the mercury-filled bulb expands into the capillary tube. Its rate of expansion is calibrated on the glass scale. Two different scales can be found on thermometers--the Fahrenheit scale and the Celsius scale.

With the Fahrenheit scale, Daniel Fahrenheit decided that the freezing and boiling points of water would be separated by 180 degrees and he pegged freezing water at 32 degrees. So he made a thermometer, stuck it in freezing water, and marked the level of the mercury on the glass as 32 degrees. Then he stuck the same thermometer in boiling water and marked it 212 degrees. He then put 180 evenly spaced marks between those two points.

In Celsius scale, Anders Celsius decided that the freezing and boiling points of water would be separated by 100 degrees and he made the freezing point of water at 100 degrees. (His scale was later inverted, so the boiling point of water became 100 degrees and the freezing point became 0 degrees.)

Bulb thermometers are most commonly found in two places--outside on our porches measuring the temperatures outside or under our tongues measuring our bodily temperatures.

With the age the technology came the invention of other types of thermometers. Each different type of thermometer has its own distinctive means of measuring or controlling temperature. For instance, bimetallic strip thermometers are extremely effective for controlling temperatures. Although bulb thermometers are good for measuring temperature accurately, they are harder to maintain set temperatures.

While bulb thermometers measure our changing temperatures when we feel feverish, bimetallic strip thermometers help us bake our favorite cakes by maintaining a set temperature in ovens. The bimetallic strip thermometer, because it is made of metal, is good at maintaining the same temperature for a long period of time.

Recent technology has created new ways to measure temperatures with electronics. The most common device is known as a thermoresistor (or thermistor). This sensor changes its resistance with changes in temperature. A computer or other electronic circuit measures the resistance and converts it to a temperature, either to display it or to make decisions about turning something on or off.

The heat is on…

The first attempt to make a standard temperature scale was done by Galen in AD 170. In his medical writings, Galen created a standard "neutral" temperature consisting of equal quantities of boiling water and ice. On each side of this "neutral" temperature were four degrees of heat and four degrees of cold.

The earliest devices that were used to measure temperature were referred to as thermoscopes. A thermoscope was a glass bulb with a long tube extending downward into a container of colored water.

Did you know?

• In 1610 Galileo supposedly used wine instead of water in thermoscopes.
• In 1641, the first sealed thermometer that used liquid rather than air as the thermometric medium was invented for Ferdinand II, Grand Duke of Tuscany. His thermometer used a sealed alcohol-in-glass device, with 50 "degree" marks on its stem but no "fixed point" was used to zero the scale. These were referred to as "spirit" thermometers.
• Robert Hook, Curator of the Royal Society, created in 1664 the thermometer that eventually became the standard temperature-measuring instrument of Gresham College and was used by the Royal Society until 1709. (The first intelligible meteorological records used this scale).
• Mercury's unique characteristics are perfect for measuring temperatures for the following reasons:
  • It has large and uniform expansion abilities,
  • Its silvery appearance allows for easy reading,
  • Its ability to remain a liquid over a wide range of temperatures.