measuring temperature, are found in some domestic kitchens and not in others; but are virtually omnipresent in large professional kitchens and food-processing establishments.
All thermometers of conventional design are descended from a device invented by the Italian scientist Galileo in 1592, and now known as a thermoscope. It was a narrow glass tube with a bulb at the bottom. This was filled with water, and the top was left open. If the temperature of the surroundings rose the water would expand, causing its level in the tube to rise. The amount of expansion was tiny, but the change in level was amplified by the fact that the bulb was much larger in diameter than the tube.
The word thermomètre was first used by the Jesuit Father Leuréchon in 1626. Soon it was realized that the device could be made more accurate by sealing it from the air so that the liquid would not evaporate. This was done by filling the tube completely, heating it till the water boiled, and then thrusting the top end into a hot brazier to melt the glass. As the water cooled it contracted, leaving a vacuum in the top of the tube. Later alcohol was used instead of water; this freezes at –114 °C (–173 °F), so that temperatures below zero can be measured. Normally it would boil at 78 °C (173 °F), but the pressure inside the tube caused by the expansion of the alcohol keeps it liquid, and alcohol thermometers can measure temperatures well above the boiling point of water. In 1670 mercury was used; this has the advantage of being much easier to see than water or alcohol, even if dye is added to these. But it freezes at –39 °C (–38 °F), so it is unsuitable for very low temperatures.
At first any scale marked on the thermometer was arbitrary. At the beginning of the 18th century the German physicist Carl Gabriel Fahrenheit built improved thermometers and devised the scale which bears his name. He fixed the zero mark (what we now call 0 °F, equal to –18 °C) as the lowest temperature he could achieve, by mixing ice and salt. The 100 °F mark was supposed to be blood heat, but he seems to have had a slight fever when he made the measurement, as normal human temperature is 98.6 °F (37 °C). In 1742 the Swedish astronomer Anders Celsius tried to produce a more rational scale, taking as the low mark the freezing point of water, and as the high mark the boiling point of water. In fact, he called the bottom of his scale 100° and the top 0°, but this was soon found inconvenient and reversed.
Glass thermometers are fragile. Where no great accuracy is needed, a bimetallic strip thermometer is often used. This contains a thin, flat strip made of a layer of iron and a layer of brass soldered together, and bent into a coil. The outer end of the coil is fixed, the inner end attached to a spindle carrying an indicator needle. Brass expands more when heated than iron does, so when the coil is heated or cooled its curvature changes, turning the spindle and moving the needle.
Thermometers are used for various purposes in cookery. The sugar thermometer is an alcohol type set in a brass holder which is designed for sugar boiling operations, including jam- and marmalade-making. It can be lowered into the hot syrup or jam (or, conveniently, fitted over the side of the pan, with its bottom end in the syrup or jam), and will register up to 180 °C (356 °F), slightly above the caramel point of sugar. It can also be used for other tasks where temperature is important, such as making yoghurt. Oven thermometers are usually of the robust bimetallic type, with a magnet on the back so that they can be fixed to the steel wall of the oven.
meat thermometers are used to tell whether the interior of a joint has reached a certain level, for example 60 °C (140 °F) for rare beef or 85 °C (185 °F) for well-done pork. One type has a sharp-ended, hollow metal probe filled with a liquid which conveys heat to a bimetallic strip thermometer at the outside end. Modern meat thermometers are electronic, with a piece of carbon inside the tip of the probe whose electrical resistance increases as it is heated. The instrument measures the resistance, converts it into a temperature, and shows the figure on a digital display.
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
