Heat: Measuring Temperature, Thermometers, Transformation of Heat

Heat: Measuring Temperature, Thermometers, Transformation of Heat

Heat, energy that is transferred from one body to another as the result of a difference in temperature. If two bodies at different temperatures are brought together, energy is transferred—i.e., heat flows—from the hotter body to the colder. The effect of this transfer of energy usually, but not always, is an increase in the temperature of the colder body and a decrease in the temperature of the hotter body.

Measuring Temperature

The measurement of temperature is a comparatively new concept. Early scientists understood the difference between “hot” and “cold,” but they had no method to quantify varying degrees of heat until the seventeenth century. In 1597, Italian astronomer Galileo Galilei invented a simple water thermoscope, a device that consisted of a long glass tube inverted in a sealed jar that contained both air and water. When the jar was heated, the air expanded and pushed the liquid up the tube. The water level in the tube could be compared at different temperatures to show relative changes as heat was added or removed. However, the thermoscope lacked an easy way to directly quantify temperature.

Several years later, the Italian physician and inventor Santorio Santorio improved Galileo’s design by adding a numerical scale to the thermoscope. These early thermoscopes led to the development of the fluid-filled thermometers commonly used today. Modern thermometers operate based on the tendency of some fluids to expand when heated. As the fluid inside a thermometer absorbs heat, it expands, occupying a greater volume and forcing the fluid level inside the tube to rise. When the fluid is cooled, it contracts, occupying a smaller volume and causing the fluid level to fall.

Temperature is a measure of the amount of heat energy possessed by an object Because temperature is a relative measurement, scales based on reference points must be used to accurately measure temperature. There are three main scales commonly used in the world today to measure temperature: the Fahrenheit (°F) scale, the Celsius (°C) scale, and the Kelvin (K) scale.

Thermometers

Thermometer is an instrument for measuring the temperature of a system. Temperature measurement is important to a wide range of activities, including manufacturing, scientific research, and medical practice.

The accurate measurement of temperature developed relatively recently in human history. The invention of the thermometer is generally credited to the Italian mathematician-physicist Galileo Galilei (1564–1642). In his instrument, built about 1592, the changing temperature of an inverted glass vessel produced an expansion or contraction of the air within it, which in turn changed the level of the liquid with which the vessel’s long, openmouthed neck was partially filled. This general principle was perfected in succeeding years by experimenting with liquids such as mercury and by providing a scale to measure the expansion and contraction brought about in such liquids by rising and falling temperatures.

By the early 18th century as many as 35 different temperature scales had been devised. The German physicist Daniel Gabriel Fahrenheit in 1700–30 produced accurate mercury thermometers calibrated to a standard scale that ranged from 32°, the melting point of ice, to 96° for body temperature. The unit of temperature (degree) on the Fahrenheit temperature scale is 1/180 of the difference between the boiling (212°) and freezing points of water. The first centigrade scale (made up of 100 degrees) is attributed to the Swedish astronomer Anders Celsius, who developed it in 1742. Celsius used 0° for the boiling point of water and 100° for the melting point of snow. This was later inverted to put 0° on the cold end and 100° on the hot end, and in that form it gained widespread use. It was known simply as the centigrade scale until in 1948 the name was changed to the Celsius temperature scale. In 1848 the British physicist William Thomson proposed a system that used the degree Celsius but was keyed to absolute zero (−273.15 °C); the unit of this scale is now known as the kelvin. The Rankine scale employs the Fahrenheit degree keyed to absolute zero (−459.67 °F).

Any substance that somehow changes with alterations in its temperature can be used as the basic component in a thermometer. Gas thermometers work best at very low temperatures. Liquid thermometers are the most common type in use. They are simple, inexpensive, long-lasting, and able to measure a wide temperature span. The liquid is almost always mercury, sealed in a glass tube with nitrogen gas making up the rest of the volume of the tube.

Electrical-resistance thermometers characteristically use platinum and operate on the principle that electrical resistance varies with changes in temperature. Thermocouples are among the most widely used industrial thermometers. They are composed of two wires made of different materials joined together at one end and connected to a voltage-measuring device at the other. A temperature difference between the two ends creates a voltage that can be measured and translated into a measure of the temperature of the junction end. The bimetallic strip constitutes one of the most trouble-free and durable thermometers. It is simply two strips of different metals bonded together and held at one end. When heated, the two strips expand at different rates, resulting in a bending effect that is used to measure the temperature change.

Transformation of Heat

Heat is transfered via solid material (conduction), liquids and gases (convection), and electromagnetical waves (radiation). Heat is usually transfered in a combination of these three types and seldomly occurs on its own. For example, the thermal environment of a building is influenced by heat fluxes through the ground (conduction), and the building envelope (mostly convection and radiation).

Convection is heat flux through liquids and gases. Heat Flux Sensors can measure convective heat flux. Examples of convective heat flux are:

Feeling much colder when it is windy.
Feeling much colder in water of 25°C than in air of 25°C.
Sensing principle in heat flux based mass flow sensors.

Conduction is heat flux through solid materials. Heat Flux Sensors can measure conductive heat flux. Examples of conductive heat flux are:

Touching a hot cup of coffee
Thermal influences in precision instruments.
Measurement of heat output from chemical reactors.

Radiation is energy that comes from a source and travels through space and may be able to penetrate various materials. Light, radio, and microwaves are types of radiation that are called nonionizing. The kind of radiation discussed in this document is called ionizing radiation because it can produce charged particles (ions) in matter.

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