Thermocouples are used to measure temperature in the range of -200,C to 2,000,C.
This of course is a general statement; the actual range varies with the type of "couple"
used.
ELECTRICAL RESISTANCE THERMOMETER
Another type of electrical thermometer is the resistance thermometer. It operates on the
principle that resistance varies as a function of heat.
If the resistance increases as temperature increases, the resistance is said to have a
positive temperature coefficient. If the resistance decreases as temperature increases, it is
said to have a negative coefficient.
The resistance thermometer in its simplest form is a series circuit consisting of a battery,
thermal resistance, ammeter, and current limiting resistor. (Refer to figure 8)
The current limiting resistor serves a dual role. First, as a calibration adjustment for the
meter and second, as current limiter to increase the useful life of the battery.
When heat is applied to a thermal element having a positive temperature coefficient, the
meter would actually indicate less current flow, but the scale face would indicate a higher
temperature level. Full scale temperature reading would occur when the thermal
resistance becomes large enough to cause the meter to reach the "zero" current level
(arbitrary minimum deflection level).
You have read about quite a few thermometers thus far in this subcourse. There are
others, but the ones already explained convey the theory that is necessary at this time.
8. TEMPERATURE SCALES.
Thus far, reference has been made to temperature on what is called the Celsius scale. We
will now discuss the various scales in common use so that they will be more meaningful.
People who attempted to measure temperature a few centuries ago had a hard time trying
to develop a scale. They did come up with some rather unique systems. For instance, in
1701, Sir Issac Newton developed a scale based on the freezing point of water (0) and
body temperature (12). In 1714, Gabriel Fahrenheit marked a mercury thermometer with
zero as a point in an ice-salt bath and 12 as blood temperature (I wonder how they
measured that?). There were many other standards such as: temperature of a deep cave,
the coldest day, etc..... It is easy to see how such references are ridiculous from our point
of view. Imagine calibrating thermometers only on the coldest day of the year.
A man named Celsius produced a scale where 0, is the freezing point (ice point) of water
and 100, the steam point of boiling water. It is divided into 100 equal parts and is called
the Celsius or centigrade (100 gradient) scale. The Celsius scale is used internationally
for precision laboratory measurements. The symbol ,C indicates degree Celsius or
centigrade. However, Celsius is the preferred designation.
In 1858, Lord Kelvin conceived a new scale which started at -273,C. It was based on the
assumption that all molecular motion ceased at this particular temperature (-273,C). This
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