Thermometers
A thermometer is any thermal sensor that measures temperature.
Thermal sensors are materials that change when they are put into
air having different temperatures. The little lines on a thermometer
are called calibrations. A thermometer can be calibrated in either
Fahrenheit or Celsius scale.
A thermometer really measures its own temperature because it becomes
the temperature of the air around it. Meteorologists use different
kinds of thermometers to measure the air near the earth and far
above the earth.
Liquid-In-Glass Thermometers
The liquid-in-glass thermometer was perfected in the 1600's.
It is the one that most people think of when they hear the word
"thermometer."
It is made of a glass tube with a bulb on one end. On the tube
are Fahrenheit or Celsius measurement marks. The glass bulb is
filled with either mercury or alcohol. When the temperature around
it changes, the liquid in the bulb and tube either expands or
contracts. If it is hotter, it will expand. This causes the
liquid to rise in the tube. If it is cooler, it will contract.
This causes the liquid to fall back down the glass tube. On
both the Fahrenheit and Celsius scales, hotter is higher and cooler
is lower.
Minimum/Maximum Thermometer
A special kind of liquid-in-glass thermometer measures the highest
and lowest temperatures in a period of time. This thermometer
is used in pairs, side by side. Once a day they are read and
then reset.
Minimum Thermometer
An alcohol thermometer is used to indicate the daily minimum temperatures
below -35 degrees Fahrenheit. A small metal index is inside the
glass tube. When the temperature reaches its lowest point, the
metal index sinks. When the temperature rises again, the metal
index stays at the lowest point and keeps a record of the lowest
temperature. After that temperature is read and recorded, the
thermometer is reset.
Maximum Thermometer
The mercury thermometer is used to indicate high temperatures.
It can record the highest temperature in a time period. There
is a constriction just above the bulb of the thermometer that
causes the mercury to rise, but when it cools, gravity doesn't
allow all of the mercury to fall. The tiny thread of mercury
breaks at the constriction at the highest temperature recorded.
Liquid-in-glass thermometers are cheap, easy to read, and easy
to maintain, but they break easily. Other thermometers are smaller,
stronger, and can be operated remotely.
Deformation Type Thermometers
Deformation thermometers use a coil made from two different kinds
of metal. These metals expand and contract at different rates
when heated. Depending upon which metal expands or contracts
the most, the amount of expansion is measured and temperature
can be read.
Deformation thermometers are used in thermostats in houses and
other buildings. They are also used in thermographs. Thermographs
are automatic temperature recording instruments. The record on
paper of temperature changes as recorded by a thermograph is called
a thermogram.
Other Thermometers
Other kinds of thermometers include electrical thermometers, such
as thermocouples, and resistance thermometers. All of these use
electricity to measure temperature. A change in the air temperature
causes a change in the electrical properties, like the electrical
current generated or the electrical resistance. These types of
thermometers are very accurate and respond rapidly to temperature
changes. These are used in instrument packages sent up on weather
balloons to measure the air temperature thousands of feet above
the ground.
Finally, a radiometer, which measures radiation, can be used as
a thermometer. The amount of radiation from a body depends on
its temperature. So, the radiometer can sense the temperature
of objects from long distances by measuring the radiation coming
from these objects.
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