Thermometer and barometer

A thermometer shows temperature as the length of a liquid thread in a fine glass tube, read against a scale fixed by melting ice and boiling water. Its companion, the barometer, weighs the air by the height of the liquid column the atmosphere can hold up: about 760 mm of mercury at sea level.

What it is

A thermometer turns temperature into a length you can read. A glass bulb full of liquid opens into a tube with a very narrow bore. When the liquid warms it expands, and its thread climbs the tube past a scale.

The barometer came out of the same Italian workshops and weighs the air above us by the height of a liquid column. With thermometers built alike, observers in different towns could compare their readings, as the first weather network did from 1654.

How it works

Liquids grow slightly when heated: ethanol by about 0.11% of its volume per degree, mercury by about 0.018%, glass far less. The bore makes this visible, because all the extra volume of the bulb must go into it. A bulb holding a fifth of a cubic centimetre of alcohol gains about 0.2 mm³ per degree; in a bore 0.2 mm across, that is a step of about 7 mm.

A scale needs points that anyone can reproduce. Pure ice always melts at the same temperature, and pure water always boils at the same temperature under the same air pressure. The Celsius scale calls them 0 and 100, with a hundred equal steps between. On high ground water boils at a lower temperature, roughly one degree less for every 300 m of altitude. Calibrating an instrument means checking it against these points or a trusted reference and noting the error; without that, two thermometers can disagree by several degrees. It matters in the kitchen: British food-safety advice asks for 70 °C held for two minutes at the centre of cooked food.

Plain water shrinks as it warms between 0 and 4 °C and freezes at 0 °C. Early sealed thermometers held alcohol, and from the eighteenth century precision instruments mostly held mercury, which stays liquid over a wide range and does not wet the glass.

Torricelli’s barometer was a glass tube about a metre long, closed at one end, filled with mercury and turned over into a bowl of mercury. The column dropped until about 760 mm stood above the bowl, with an almost empty space above it. Its weight balances the push of the air on the open surface. Mercury is 13.6 times denser than water, so a water barometer needs more than 10 m of tube, the same height that limits a suction pump.

Near sea level the column falls by about a millimetre for every 11 m you climb, so a barometer also measures height. A falling barometer often comes before wind and rain, a rising one before fair weather. Aneroid barometers use a sealed metal capsule that flexes instead of a liquid.

How to build

The liquid thermometer is a teaching instrument: compare it with a reference to learn its error, and never use it for food or to check a fever. The jar barometer only shows whether the pressure is rising or falling.

The lower fixed point: the bottle stands in melting ice until its thread stops moving.

The liquid thermometer. Take a small glass bottle of about 100 ml, a clear drinking straw at least 25 cm long and modelling clay. Mix equal volumes of water and 70% isopropyl alcohol (rubbing alcohol), tinted with food colouring: the mix expands more than water and avoids its odd behaviour near freezing. Fill the bottle to the brim, push the straw well into the liquid and seal the neck with clay, leaving no air bubble. At room temperature the liquid should stand about 8 cm above the clay; add or remove a little through the straw with a dropper. The larger the bottle and the narrower the straw, the bigger the step per degree. Tape a paper strip behind the straw.

Fix the points. Stand the bottle in a bowl of crushed ice and a little water, deep enough to cover it up to the neck. Stir, wait until the thread stops moving, which can take a quarter of an hour, and mark 0; the liquid must still show above the clay. Let it come back to room temperature, then put it and a digital kitchen thermometer side by side in a bowl of warm tap water at about 40 °C, stir, wait until both readings stop moving and mark the reference’s reading; the thread must not reach the top. If either check fails, change the amount of liquid or the bottle and start both points again. Divide the gap into equal steps and compare with the reference at room temperature to find your error. Never squeeze the bottle.

A jar barometer: the membrane sinks when the pressure rises, and the straw tip lifts.

The jar barometer. Cut the neck off a balloon, stretch it tight over a wide glass jar and tie it with string so that no air passes. Glue one end of a long straw to the centre of the membrane and let the other overhang the rim, in front of a card. When the outside pressure rises, the membrane sinks and the tip lifts. The trapped air also swells with heat: one degree moves the pointer about as much as 3.5 hPa of pressure. Keep the jar out of the sun in a room whose temperature changes little, and compare its movements with the pressure trend from your weather service.

Reading. Keep your eye level with the top of the thread.

Safety. Never build an instrument with mercury: its vapour is toxic. If an old mercury thermometer breaks, get people and pets out and keep them out, open windows to the outside and shut doors to the rest of the house. Do not vacuum or sweep. On a hard floor, wearing gloves, gather the beads with stiff card, lift them onto a damp paper towel, pick up tiny ones with sticky tape, and seal it all in a labelled bag. On carpet or upholstery, cut out or set aside the soiled part and ask your local health or environmental service. Once the cleanup is done, keep the room aired for at least 24 hours, and take the waste to a hazardous-waste collection as your local authority directs. Isopropyl alcohol is flammable and poisonous: keep it from flames and children. Use hot tap water, never boiling water, and never move glass straight from ice to hot water.

History and evolution

Galileo’s thermoscope: as the air in the flask cools, water rises in the neck.

Philo of Byzantium and Hero of Alexandria knew that warmed air swells and pushes water along a tube. Benedetto Castelli later recalled Galileo showing such a thermoscope around 1600, and by 1612 Santorio, a physician in Venice, had given it a scale to follow fevers. Being open, it also reacted to air pressure.

Torricelli described the mercury barometer in a letter of June 1644. In 1648 Pascal’s brother-in-law Florin Périer carried one up the Puy de Dôme, where the column stood about 8.5 cm lower than in Clermont below. In Florence, makers working for Grand Duke Ferdinand II sealed alcohol in glass, and in 1654 the duke sent matching thermometers to the first weather network.

Daniel Gabriel Fahrenheit made mercury thermometers in Amsterdam from the 1710s and published his method in 1724. Anders Celsius proposed his scale in 1742 with 0 at boiling and 100 at freezing; it was turned round soon after his death in 1744. In 1848 William Thomson, later Lord Kelvin, proposed an absolute scale, its zero now set at −273.15 °C.

In 1868 Carl Wunderlich, drawing on about 25,000 patients, put the normal mean at about 37 °C and the start of fever near 38 °C. Robert FitzRoy began British storm warnings in 1861 and published the first public forecast in The Times that August. Digital sensors and thermometers filled with other liquids have since largely replaced mercury thermometers, which many countries now restrict.