Thermometer ................. The thermometer is an instrument for measuring temperature whose development began in the early seventeenth century. Thermometers are based on the property of gases, liquids, and solids to expand or contract with changes in temperature. Other thermometer designs are based on electrical phenomena. Liquid thermometer Thermometer in which temperature variations are indicated by changes in the volume of a liquid. The latter is placed in a bulb, and expands (or contracts) in a capillary tube. The liquid—alcohol, mercury, toluol, etc.—was chosen in accordance with the range of temperatures to be measured. Metal thermometer Thermometer in which temperature variations are generally indicated by the changes in length of one or more metal rods. Bimetal thermometer Thermometer in which temperature variations are indicated by the changes in the shape of a bar consisting of two metals with different expansion coefficients soldered together. Flat bimetallic plates curl when they experience temperature variations; coiled bimetallic plates unfold or fold tighter. Recording thermometer One of several kinds of thermometer (liquid, metal, etc.) whose readings are recorded by a pen on a clockwork-driven drum carrying a ribbon of graduated paper. Also called self-registering thermometer. Maximum and minimum thermometer Thermometer in which the maximum and minimum temperatures reached in a given time interval are memorized by means of various devices, such as a quantity of liquid poured into a vessel, or the displacement of tiny cursors in the capillary tubes. Gas thermometer Thermometer in which temperature variations are indicated by the expansion and contraction of a gas. Laboratories use this type of apparatus for precision measurement. Electrical thermometer Thermometer indicating temperature variations by means of electrical current flowing through a circuit in which a galvanometer is inserted. The sensitive element can be an electrical resistance whose value changes with temperature, or a thermocouple (formed by two soldered metals), which also generates specific quantities of current at different temperatures. Differential thermometers Thermometers capable of measuring or indicating the (generally small) difference between two temperatures. Often called thermoscopes, they were commonly used to study thermal radiation. They consist of a capillary U-tube closed at both ends by two glass bulbs. In the tube is inserted a drop of liquid serving as an indicator. When one of the two bulbs (which is often blackened) is exposed to a source of thermal radiation, the air in the bulb expands, pushing the drop along the capillary tube toward the second bulb. Thermometers calibration Thermometer can be calibrated either by comparing them with other certified thermometers or by checking them against known fixed points on the temperature scale. The best known of these fixed points are the melting and boiling points of pure water. (Note that the boiling point of water varies with pressure, so this must be controlled.) The traditional method of putting a scale on a liquid-in glass or liquid-in-metal thermometer was in three stages: 1. Immerse the sensing portion in a stirred mixture of pure ice and water and mark the point indicated when it had come to thermal equilibrium. 2. Immerse the sensing portion in a steam bath at one standard atmosphere (101.325 kPa = 760 mm Hg) and again mark the point indicated. 3. Divide the distance between these marks into equal portions according to the temperature scale being used. Other fixed points were used in the past are the body temperature (of a healthy adult male) which was originally used by Fahrenheit as his upper fixed point (96 degrees F to be a number divisible by 12) and the lowest temperature given by a mixture of salt and ice, which was originally the definition of 0 degrees Fahrenheit. (This is an example of a Frigorific mixture). As body temperature varies, the Fahrenheit scale was later changed to use an upper fixed point of boiling water at 212 degrees. These have now been replaced by the defining points in the International Temperature Scale of 1990, though in practice the melting point of water is more commonly used than its triple point, the latter being more difficult to manage and thus restricted to critical standard measurement. Nowadays manufacturers will often use a thermostat bath or solid block where the temperature is held constant relative to a calibrated thermometer. Other thermometers to be calibrated are put into the same bath or block and allowed to come to equilibrium, then the scale marked, or any deviation from the instrument scale recorded. For many modern devices calibration will be stating some value to be used in processing an electronic signal to convert it to a temperature. Medical thermometer A medical/clinical thermometer showing the temperature of 38.7 °C Medical thermometers are used for measuring human body temperature, with the tip of the thermometer being inserted either into the mouth (oral temperature), under the armpit (axillary temperature), or into the rectum via the anus (rectal temperature). Classification, by technology Electronic clinical thermometer The traditional mercury-filled medical thermometer works in the same way as a meteorological maximum thermometer. The thermometer consists of a mercury-filled bulb attached to a small tube. There is a constriction in the neck close to the bulb. As the temperature rises, the force of the expansion pushes the mercury up the tube through the constriction. When the temperature falls, the column of mercury breaks at the constriction and cannot return to the bulb, thus remaining stationary in the tube. To reset the thermometer, it must be swung sharply. When it is designed for use in humans, the typical range of this kind of thermometer is from about 89.6°F to 109.4°F or 35°C to 42°C. The temperature is obtained by reading the scale inscribed on the side of the thermometer. Close-up of a maximum thermometer. The break in the column of mercury is visible. In the 1990s, mercury-based thermometers were found too risky to handle and have largely been replaced with electronic digital thermometers, or, more rarely, thermometers based on liquids other than mercury (such as heat- sensitive liquid crystals). Other modern options include digital Infrared contact or non-contact thermometers, which are also called scanner thermometers. Most medical thermometers may be used to take oral, axillary, vaginal, or rectal temperatures. To eliminate the risk of patient cross-infection, disposable single-use clinical thermometers and probe covers are employed in clinics and hospitals. Thermoscope At the start of the seventeenth century there was no way to quantify heat. In Aristotelian matter theory, heat and cold were fundamental qualities. Like dry and wet, heat and cold were qualities combined with "prima materia" to make up the elements, earth, water, air, and fire. Thus earth was dry and cold, fire dry and hot, etc. Although one might speak of "degrees of heat or cold," there was no formal distinction between what we would call the extensive concept of heat and the intensive concept of temperature. Also these degrees were not measured, except perhaps in a very rough way as when a physician put his hand on a patient's forehead and diagnosed "fever heat." Measuring heat became a puzzle in the circle of practical and learned men in Venice to which Galileo belonged. The first solution was a thermoscope. Building on Pneumatics by Hero of Alexandria (1st century BCE), first published in the West in 1575, several authors had begun playing with the idea of the expansion of air as its heat increased, and vice versa. The first versions, usually called thermoscopes, were little more than toys. Benedetto Castelli wrote in 1638 about a device he had seen in Galileo's hands around 1603: Thermoscope He took a small glass flask, about as large as a small hen's egg, with a neck about two spans long [perhaps 16 inches] and as fine as a wheat straw, and warmed the flask well in his hands, then turned its mouth upside down into the a vessel placed underneath, in which there was a little water. When he took away the heat of his hands from the flask, the water at once began to rise in the neck, and mounted to more than a span above the level of the water in the vessel. The same Sig. Galileo had then made use of this effect in order to construct an instrument for examining the degrees of heat and cold. Over the next several years this thermoscope was developed by Santorio Santorio and Galileo's friend Gianfrancesco Sagredo (both in Venice), Galileo, and others to include a numerical scale. It had thus become a full-fledged air thermometer. The first series of quantitative meteorological observations date from this period. In other parts of Europe the inventor Cornelis Drebbel and Robert Fludd developed similar instruments. The questions about who was the first, and whether one derived his knowledge from another, are sterile ones which shed little light on the historical context in which this and other instruments (e.g., the telescope and barometer) developed. The near simultaneous (and surely independent) invention of the air thermometer illustrates the seventeenth-century trend toward quantification of natural phenomena--an essential dimension of the "mathematization of nature." The liquid in glass thermometer was developed in the 1630s, but a universal standard of temperature remained elusive. Each scientist had his own scale divisions, often based on different reference points. It is impossible for us accurately to convert their measurements to our temperature scale, and at the time it was impossible to compare temperatures in different places. In the early eighteenth century, universal temperature scales based on several fiduciary points (e.g. a mixture of ice and brine, a mixture of ice and ??? water, body temperature, the boiling point of water) were developed by Daniel Gabriel Fahrenheit (1686-1736), Anders Celsius (1701-1744), and René-Antoine Ferchault de Réaumur (1683-1757). Of these, the first two are still in use, and the system of Celsius (extended to become an absolute scale in the nineteenth century) has become the standard scientific temperature scale.
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