Science, Maths & Technology

# Challenge: Make a Thermometer

Updated Monday 28th January 2008

The science behind measuring temperature, part of the BBC/OU's programme website for Rough Science 2

There are different kinds of thermometers, such as electronic, glass, resistance and gas. Which of these would the Rough Scientists attempt to make with just the most basic equipment?

Firstly, what are we trying to measure?

Temperature is a measurement of how hot or cold a substance is. Temperature indicates how much energy the atoms inside the substance have. The more energy the atoms have, the faster they move and the hotter the substance.

For each substance, there are specific temperatures at which changes of state happen. These are known as melting and freezing points. Absolute zero (-273°C) is the coldest temperature that we can get to and represents the temperature at which all movement of atoms should stop.

Copyright: Used with permission Temperature is measured in degrees Centigrade (or Celcius) by thermometers.

How can we measure temperature?

The earliest devices used to measure temperature were called thermoscopes. They consisted of a big glass bulb with a long tube extending downwards into a container of coloured water or wine.

Some of the air in the bulb was expelled before placing it in the liquid, causing the liquid to partially rise into the tube. As the remaining air in the bulb was heated or cooled, the level of the liquid in the tube would vary, reflecting the change in the air temperature. An engraved scale on the tube provided a way of measuring the fluctuations.

The most common kind of thermometer consists of a liquid, such as alcohol or mercury, in a very thin, hollow glass tube. It works because of the fact that liquids expand more than solids as their temperatures increase. If both the liquid and glass expanded equally, the whole thermometer would simply increase in size. Instead, what actually happens is that the liquid expands more than the glass and moves up the capillary, registering a change in temperature.

Thermometers are usually calibrated with a certain range of temperatures in mind. For example, a clinical thermometer used to measure human body temperature would only have measurements ranging from 32°C to 42°C.

What type of thermometer did the scientists make on the island?

The scientists actually made two thermometers. Firstly, a glass thermometer was constructed using methanol as the liquid. A very thin tube was made by heating glass in a furnace and then carefully stretching the molten glass.

In order to calibrate the thermometer, we needed several known temperatures such as ice (0°C) or boiling water (100°C). We couldn't use either: there was a distinct lack of ice on our Caribbean island and methanol boils at 67°C so would have completely evaporated by the time the water reached 100°C. Instead we use sea water (which we guessed to be around 24°C) and room temperature (which we thought to be around 30°C in the lime factory).

The second type of thermometer made was a resistance thermometer.

This device uses changes in the electrical resistance of a metal to measure temperature differences. We made a voltmeter then put the metal to be tested within a Wheatstone Bridge circuit which enables you to measure changes in electrical resistances very accurately and hence temperature differences.

Both the glass thermometer and resistance thermometer were successful, however, the glass thermometer was more sensitive to changes in temperature.

The BBC and the Open University are not responsible for the content of external websites.

How Thermometers Work - from the How Stuff Works website

Thermometers and Thermostats - by Louis A. Bloomfield on the How Things Work, Virginia University site

About Temperature - from the Unidata site

Wheatstone Bridges - from the Engineering Fundamentals site

The Wheatstone Bridge - from a site about Sir Charles Wheatstone

## Books

Advanced Physics by Tom Duncan, John Murray

A History of the Thermometer and Its Uses in Meteorology by W. E. Knowles Middleton, John Hopkins University Press

The Construction of a Thermometer by James Six, Nimbus Publishing Ltd

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