Thermometer - DOC by asafwewe

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									                                   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[12]. (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[14]. 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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