### Thermometers and Scales.

Most people think of thermometers as long thin glass tubes filled either with mercury or with some red liquid. They know that body temperature is either 96.8 ° F or 37 ° C, and have some familiarity with other common temperatures such as the freezing point of water, a warm day, or a cup of hot coffee.

The 'common' thermometer that tells us these values is a descendant of a piece of equipment devised originally by Galileo in about 1592. This first thermometer consisted of an inverted glass vessel with a long openmouthed neck, partially filled with liquid. As the temperature increased the air in the vessel expanded or contracted and the level of the liquid changed accordingly.

Temperature measurement, therefore, began by using a general principle; that of observing the expansion (in size) of a substance with changes in temperature. The technique was perfected in succeeding years by experimenting with the expansion of liquids as they grew hotter. As the temperature increased, the liquid would expand along a very thin tube. The amount of the expansion determined the height of the temperature. The 'degree' of expansion was determined by a 'scale' of values scored along the side of the tube.

At one point in 18th century there were as many as 35 different temperature scales. But in the succeeding decades the one devised the German physicist Daniel Gabriel Fahrenheit became 'the' standard. It had a scale that ranged from the lowest temperature he could reach with a mixture of ice and salt (0 ° F) to melting point of ice (which Fahrenheit set at 32 °), to the body temperature of his wife (which he originally set at 100 ° F, but she must have had a fever that day, as body temperature is now set at 98.6 ° F).

A degree on the Fahrenheit scale is 1/180 of the range between the boiling point and freezing point of water. This is relatively small value and an awkward one for the purposes of calculations. The first scale that divided this range into a 100 units was the one devised by the Swedish astronomer Anders Celsius. In 1742 Celsius set the boiling point of water at 0 ° and the melting point of snow at 100 °. No this is not a typing mistake, this was the original suggestion, but it was later inverted to make 0 ° the colder value and 100 ° the hotter value, which is the way we use it today. For a while this was known simply as the centigrade scale ('centi' = 100) until 1948 when the name was changed to "Celsius" in honor of its inventor.

The battle of temperature scales heated up in 1848 when the British physicist William Thomson (Lord Kelvin) discovered the fact that there was a temperature below which it was impossible to go. He proposed a system based on the 'degrees Celsius' but which started at this coldest temperature, absolute zero (-273.15 ° C). Not commonly used outside a science laboratory the unit of this scale is the kelvin. An equivalent Rankine scale uses the Fahrenheit 'degree', which makes absolute zero (-459.67 ° F).

Although liquid thermometers are the most common, other types of temperature measuring devices are available. Thermometers that depend on the change in volume of gasses are used to measure very low temperatures. Electrical resistance changes with temperature, and thermometers can be made that use platinum and way electrical resistance varies with increases or decreases in temperature.

If two wires, made of different materials, are joined together at one end and then connected to a volt-meter any temperature difference can be measured by observing the difference in voltage produced in the thermocouple. A 'bimetallic strip' is one of the most rugged, trouble-free and least easily damaged thermometers. Made of two strips of different metals bonded together and held at one end, when it is heated, the two metals expand by different amounts. Because they are held together, the strip bends into a 'banana" shape, and the amount of the bending can be used to measure the amount of temperature change.

More esoteric thermometers make use of sound waves, magnetic fields (for very, very cold temperatures), and by measuring the amount if infra-red radiation radiating away from an object.

Science@a Distance
© 2001, Professor John Blamire