It’s a hot day, a really hot day. But is it hotter than yesterday? The idea that temperature is a quantifiable property and that we can measure it is a relatively modern one. In the ancient world, natural philosophers regarded heat as a quality of matter. Indeed, the distinction between heat and temperature continues to elude many students. It wasn’t until the 17th century that reliable instruments were developed which were capable of discriminating between temperatures.

Without a thermometer, we are reduced to making estimates based on our own senses. There are some common sources of evidence - those days when your shoes feel tight, the UPVC windows won’t open, and tabloid journalists throng the pavements frying eggs. The problem is not that the temperature is high, but rather just how high it is. Is it hotter than yesterday, or than last week or last century?

Galileo is often credited with the invention of the first thermoscope, a simple air thermometer. The changes in the volume of air were used to indicate changing temperature. Unfortunately, air volume is also affected by changes in air pressure, so it’s not a reliable guide to temperature changes. You wonder if Galileo really understood about fair tests.

It was soon realised that thermometers must be sealed, both to prevent evaporation of the liquid inside them and to isolate them from changes in external pressure.

The first mercury-filled thermometer was made in 1659, by an astronomer, Ismael Boulliau. But without an agreed temperature scale there were still problems about comparing temperatures. Early thermometers used a variety of fixed points, which made temperature comparisons difficult. In Florence, summer and winter temperatures were taken as the high and low points - quite a variability here. Others used the melting point of butter or even the body temperatures of animals.

Even when a thermometer is working well, there can still be sampling errors. In the 19th century, ships measured the surface temperature of the sea each day. The problem was that some sailors used wooden buckets while others used canvas ones. As canvas provides less insulation than wood, the water temperature could change more quickly in a canvas bucket before it could be measured.

Mercury thermometers work because the liquid metal expands on heating.

Don’t forget that the glass tube also expands, and not necessarily at the same rate as the mercury. You could use the height of the Eiffel Tower as a thermometer - on hot days it must grow by many centimetres. There are more accessible ways to use expansion to measure temperature changes.

If mercury solved the problem of measuring earthly substances, what could be done about objects farther away? Measuring the temperatures of stars presents obvious problems. One clue comes from the effect seen with dimmer switches, or variable resistors.

And what of very high temperatures in industrial processes? Anyone who has thrown a pot and then fired it understands the importance of temperature control in the kiln. Josiah Wedgwood (1730-1795), of the famous pottery company, needed a way to measure kiln temperatures, which were far beyond the range of existing thermometers. He developed a thermometer that used a very unusual idea: a small cylinder of clay, which contracted as it was heated. The degree of contraction indicated the temperature in the kiln, but the device was not very reliable. Ask students to use examples such as this to search the internet for details of unusual thermometers. For example, they could start with the fever strips that use liquid crystal displays, or battery life indicator strips on the sides of batteries.

References

www.howstuffworks.comtherm.htm

www.Kapili.comphysics4kidsthermoindex.html

* The following books are out of print, but available in libraries: The Climate Revealed by William J Burroughs (Mitchell Beazley) A Field Guide to Stars and Planets by DonaldMenzel (Houghton Mifflin) The Universe by Iain Nicolson, Patrick Moore (HarperCollins)

EXPERIMENTS

1 Galileo’s thermoscope (KS32)

* You will need an empty 50ml bottle or flask fitted with an air-tight stopper and long straight tube.

* Set up the bottle with the tube pointing downwards and the end dipping into some coloured water.

* Warm the bottle to expand the air inside, letting the excess bubble out.

As it cools, the air contracts and coloured water rises up the tube.

* Further temperature variations cause the liquid level to rise or fall.

* Galileo’s first thermoscope did not have a temperature scale. Ask students to devise a scale, using an ordinary thermometer as reference.

* Investigate the reliability of the thermoscope and identify any drawbacks in its use.

2 How fixed is a fixed point?

* Use an ordinary glass thermometer to check the fixed points of melting ice (0C) and boiling water (100C).

* Place crushed ice in a plastic funnel, with the thermometer bulb surrounded by melting ice.

* Try pure ice and then ice-salt mixtures (salt lowers the freezing point of water).

* Suspend a thermometer just above the surface of water boiling in a conical flask.

* Try pure water and then salty water (the boiling point goes up).

* Ask students about the reliability of these fixed points.

3 The perfect temperature measurement

* Set the challenge of making reliable air temperature measurements over several days.

* Students need to choose the location carefully. Think about: sunshine or shade; indoors or out; winds and draughts; time of day or night; one thermometer or more.

4 Different types of thermometer

The roller thermometer

* Clamp one end of a 20cm metal rod and lightly rest the other end on a knitting needle, to act as a support roller. Attach a paper pointer to the needle.

* Try heating the centre of the metal rod using a candle or gas flame. As the rod expands, the roller and pointer will turn.

* Devise a scale that will distinguish between two sources of heat.

The wire thermometer

* Stretch a thin copper wire, about 1 metre long, between two points.

Suspend a pair of scissors from the centre of the wire.

* Warm the wire using a candle or other flame. As it heats up, the wire lengthens and the scissors move downwards.

* Devise a scale that uses the position of the scissor point as the reading. Is this a practical thermometer?

* Design a wire thermometer based on a coil of wire with a pointer attached. The pointer must turn as the wire coil changes temperature.

The bubble thermometer

* Fit a bottle or flask with a stopper and narrow tube, about 6cm long.

* Place a drop of bubble mixture or soapy water in the end of the tube.

* Place the bottle in a warmer place and watch a bubble form on the end of the tube.

* See what happens as the bottle is alternately warmed and cooled. The size of the bubble could be used to indicate temperature.

* What are the drawbacks of using this as a thermometer?

5 The luminous thermometer (KS43 )

* Set up a low voltage circuit with a clear glass lamp, variable resistor and ammeter.

* Investigate the connection between the current and the colour of the lamp filament. It will change from dull red to white hot as its temperature rises.

* The colour of a star is a guide to its temperature. The higher the temperature of the star, the shorter the wavelength of the light. Very hot stars appear to be white or blue-white, while cooler stars look red.

Star colour Star temp (C)

Very blue 50,000

Green 11,000

Yellow 6,000

Red 3,000

You can deduce the surface temperature of the Sun from this data - it is about 6,000C.