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As in any technical discussion, keeping the terminology straight is important. Refer to this glossary if things start to get confusing.

Heat is the amount of energy moved across a thermodynamic barrier, and is measured in Joules (J).

Heat transfer is the rate at which energy moves across the thermodynamic barrier, measured in Watts (W), that is, Joules per second. Heat transfer occurs in three different modes, conduction, convection, and radiation.

Heat flux is the rate of energy transfer per unit area, expressed in W/m?, W/cm?, or similar units. Heat flux can be positive or negative, depending on the direction of heat transfer.

Temperature is a fundamental property that indicates the internal energy of matter. Any temperature scale (Celsius, Fahrenheit, etc.) may be used as long as the units are kept consistent.

Conduction is a mode of heat transfer through a substance, either solid or fluid, on a molecular level as a result of a temperature gradient being present.

Convection is a mode of heat transfer when there is fluid flow. As in conduction, a temperature gradient must be present, but convection is influenced by fluid flow, which alters the temperature gradient.

Radiation is a mode of heat transfer that occurs via electromagnetic radiation, and does not require any transport medium or material.

Emissivity is the ratio of the actual energy emitted by a real body to that emitted by a blackbody. Emissivity can be considered equal to absorptivity for a gray body; that is a body whose emissivity is independent of wavelength and which reflects radiation in a diffuse manner. Most objects can be reasonably approximated as gray bodies.

A thermocouple is used to measure temperature. It consists of a pair of junctions between two different metals that will produce a voltage proportional to the temperature difference between the junctions of the wires due to the Seebeck effect. Commercially available thermocouples will appear to have only one junction; the second junction is essentially where the two leads are connected to a voltmeter or electronic thermometer. For a thermocouple to give an accurate reading the second junction must be at a reference temperature; frequently this is taken to be room temperature. For more accurate measurements, the second junction is lowered to a known temperature, such as the ice point.

A thermopile is an array of thermocouples. By connecting many thermocouples in series, the temperature sensitivity is increased, because the thermocouple voltages add when linked in series. Like a thermocouple, the thermopile reads the temperature difference between two points. For a heat flux transducer, these two points are the top and bottom layers of the thermopile.