<P> A basic melting point apparatus for the analysis of crystalline solids consists of an oil bath with a transparent window (most basic design: a Thiele tube) and a simple magnifier . The several grains of a solid are placed in a thin glass tube and partially immersed in the oil bath . The oil bath is heated (and stirred) and with the aid of the magnifier (and external light source) melting of the individual crystals at a certain temperature can be observed . In large / small devices, the sample is placed in a heating block, and optical detection is automated . </P> <P> The measurement can also be made continuously with an operating process . For instance, oil refineries measure the freeze point of diesel fuel online, meaning that the sample is taken from the process and measured automatically . This allows for more frequent measurements as the sample does not have to be manually collected and taken to a remote laboratory . </P> <P> For refractory materials (e.g. platinum, tungsten, tantalum, some carbides and nitrides, etc .) the extremely high melting point (typically considered to be above, say, 1800 ° C) may be determined by heating the material in a black body furnace and measuring the black - body temperature with an optical pyrometer . For the highest melting materials, this may require extrapolation by several hundred degrees . The spectral radiance from an incandescent body is known to be a function of its temperature . An optical pyrometer matches the radiance of a body under study to the radiance of a source that has been previously calibrated as a function of temperature . In this way, the measurement of the absolute magnitude of the intensity of radiation is unnecessary . However, known temperatures must be used to determine the calibration of the pyrometer . For temperatures above the calibration range of the source, an extrapolation technique must be employed . This extrapolation is accomplished by using Planck's law of radiation . The constants in this equation are not known with sufficient accuracy, causing errors in the extrapolation to become larger at higher temperatures . However, standard techniques have been developed to perform this extrapolation . </P> <P> Consider the case of using gold as the source (mp = 1063 ° C). In this technique, the current through the filament of the pyrometer is adjusted until the light intensity of the filament matches that of a black - body at the melting point of gold . This establishes the primary calibration temperature and can be expressed in terms of current through the pyrometer lamp . With the same current setting, the pyrometer is sighted on another black - body at a higher temperature . An absorbing medium of known transmission is inserted between the pyrometer and this black - body . The temperature of the black - body is then adjusted until a match exists between its intensity and that of the pyrometer filament . The true higher temperature of the black - body is then determined from Planck's Law . The absorbing medium is then removed and the current through the filament is adjusted to match the filament intensity to that of the black - body . This establishes a second calibration point for the pyrometer . This step is repeated to carry the calibration to higher temperatures . Now, temperatures and their corresponding pyrometer filament currents are known and a curve of temperature versus current can be drawn . This curve can then be extrapolated to very high temperatures . </P>

What does having a high melting point mean