Thermometers: Understanding Temperature Measurement

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Introduction to Thermometer

Thermometer, an instrument for measuring temperature. The measurement is always indirect—that is, it is a measurement not of temperature itself but of the effect of temperature on some physical property of a material used in the thermometer. In response to changes in temperature, for example, solids, liquids, and gases expand and contract; the pressure of a gas that is kept at a constant volume changes; and the electrical resistance of a metal or alloy increases and decreases.

Most thermometers have scales marked off in degrees, the units in which temperature is measured. The most common scales are:

Fahrenheit,

used in everyday temperature measurement in the United States. On this scale, the freezing point of water is 32 and its boiling point 212. (The zero point was assigned to the temperature of a mixture of ice and salt, which in the early 1800's was the lowest temperature attainable in the laboratory.)

Rankine,

used primarily by American engineers. Like the Fahrenheit scale, the Rankine scale has a difference of 180 degrees between the freezing point (491.7 R.) and boiling point (671.7 R.) of water, but the zero point is absolute zero (–459.7 F.), the lowest temperature theoretically possible.

Celsius, or Centigrade,

used generally throughout the world and in scientific work. On the Celsius scale, the freezing point of water is O and its boiling point 100. (The common name centigrade derives from the 100-degree difference between the freezing and boiling points of water.)

Kelvin,

used primarily by physicists working with very low temperatures. The unit of temperature on the Kelvin scale is called the kelvin. It is equal to a Celsius degree. The zero point of the Kelvin scale, like that of the Rankine scale, is absolute zero (–273.15 C).

Raumur,

used to a limited extent in parts of Europe. On the Raumur scale, the freezing point of water is O and its boiling point 80.

Fahrenheit to Celsius:C 59 (F. - 32)Celsius to Fahrenheit:F. (95C.) + 32Fahrenheit to Rankine:R.F. + 459.7Rankine to Fahrenheit:F. R. - 459.7Celsius to Raumur:R. 45C.Raumur to Celsius:C. 54R.Celsius to Kelvin:KC. + 273.15Kelvin to Celsius:C. K - 273.15

Kinds of Thermometers

Liquid-in-glass Thermometers

The most common type of thermometer is the liquid-in-glass thermometer. It consists of a narrow, sealed glass tube with a bulb filled with mercury, alcohol (usually dyed red, to make it easily visible), or some other liquid at its lower end. As the temperature rises, the liquid expands and rises in the tube. A drop in temperature causes the liquid to contract, so that its level in the tube falls.

Bimetal Thermometers

A bimetal thermometer contains a strip consisting of two fused strips of different metallic substances (metals or alloys). The strip is usually in the form of a spiral or rod fixed at one end. When the temperature changes, one of the metallic substances expands or contracts more than the other, causing the strip to wind or unwind, or to bend. A pointer attached to the free end of the strip indicates the temperature. Bimetal strips are widely used in dial thermometers and in thermostats. Another type of thermometer using dissimilar metals is the thermocouple.

The thermograph, or recording thermometer, which provides in chart form a continuous record of temperature during a desired period of time, is often of the bimetal type. A pen or stylus attached to the free end of the bimetal strip records the temperature on a chart fastened to a rotating disk or drum.

Resistance Thermometers

determine temperature by measuring the change in electrical resistance of a metal wire or of a thermistor, a resistor composed of a semiconducting material. The wire or thermistor is usually enclosed in a slender rod, or probe. The temperature is indicated on a meter or as a digital display.

Infrared Thermometers

measure the infrared (heat) radiation given off by an object to determine its temperature. The pyroelectric thermometer contains a type of crystal that produces an electrical signal whose strength is proportional to the infrared radiation to which it is exposed. The most familiar use of such thermometers is in ear thermometers that measure infrared radiation from the tympanic membrane (ear drum) to determine body temperature. Other types of infrared thermometers include bolometers and pyrometers.

Gas Thermometers

The most common type, the constant-volume gas thermometer, determines temperature by measuring the pressure needed to keep the volume of a gas constant as the temperature varies. The pressure of a gas confined to a constant volume varies with temperature in a known manner; if the pressure of the gas is measured, its temperature can be easily calculated.

Liquid-crystal Thermometers

contain liquid crystals that change color over a certain range of temperatures. Various mixtures of liquid crystals are used, each mixture in a separate container and each becoming light in color at a specific temperature. In digital types, each container is covered with a stencil of a number that corresponds to the temperature at which the mixture in the container becomes light in color. Fever thermometers designed to be placed on the forehead and most indoor digital thermometers are of this type.

History

The Italian scientist Galileo Galilei invented the first thermometer in 1593. His instrument, which he called a thermoscope, consisted of an air-filled glass bulb with a narrow glass tube, the end of which was open and immersed in a container of colored water. Variations in the temperature of the air in the bulb caused the water to rise or fall in the tube, on which a scale was marked.

In later thermometers, Galileo's arrangement was reversed; the bulb was filled with a liquid, usually water or alcohol, and the tube was open to the air. The first sealed thermometers, using alcohol, were developed about 1641 by Ferdinand II, the Grand Duke of Tuscany. The mercury thermometer was invented in 1714 by the German physicist Gabriel Fahrenheit.

The scale developed by the French physicist Ren Antoine Raumur in 1731 was the first to use the freezing point of water as zero. The Swedish mathematician Anders Celsius in 1742 proposed the 100-unit scale. Research in thermodynamics led to the Rankine scale (named in honor of the Scottish engineer and physicist William J. M. Rankine) and the scale developed by William Thomson, later Baron Kelvin.