|Science Focus Topic 2 Notes: Measuring Temperature||| Print ||
A relative idea about temperature is that it tells you how hot or cold something is. This can
be done by using our senses:
Touch (sensitive nerve endings on your skin can detect changes in temperature)
Sight (the color of the material giving off heat)
Relative ways to determine the temperature are not always reliable or safe.
Thermometers are more reliable devices that measure temperature
The Italian scientist Galileo invented the first air thermometer around 1600 and it has, and will continue to be, improved upon.
Early thermometers (like the one Galileo invented) did not have any scale (markings with numbers) to determine precise temperature.
The 1st precise scale was developed by Anders Celsius in 1742. He used 'degree' as the unit of temperature. All of his standards for comparison to make his markings (on his scale) were based on the properties of water.
Ø 0o was assigned the temperature at which ice melts at sea level
Ø 100o was assigned the temperature at which liquid water boils at sea level
Ø The region between (above and below, as well) these two extremes was separated into 100 equal units (degrees)
Ø The two fixed temperatures that Celsius chose can be used to calibrate a thermometer (p. 195)
Pressure also affects the freezing and boiling points of water. Extremely high pressure can cause ice to melt at a temperature below 0o (Ice skaters actually glide on a thin layer of water). Low pressure enables water to boil at a temperature below 100o. (On top of Mt. Everest, water boils at 69o)
Absolute zero is the coldest possible temperature - 273o and is used by scientists. The Kelvin scale was developed by William Thomson - a.k.a. Lord Kelvin - and the markings on the scale are not called degrees, but are simply called kelvins.
(0o Celsius is equal to 273.15o Kelvin)
The Right Device for the Job
Measuring different extremes of temperatures means using different types of devices to measure these extremes. The thermometers used for this purpose have:
A sensor - a material which is affected by changes in some feature of the environment, such as temperature
A signal - provides information about the temperature, such as an electric current
A responder - which indicates the data with a pointer, light or other mechanism using the signal
Two wires of different metals are twisted together. When heat is applied to one end an electric current is produced. (the amount of current depends on the temperature and the type of wires) This current can turn on and off a switch or valve.
The Bimetallic Strip
A bimetallic strip is made of two different metals joined (fused) together, often formed into a coil. When heat is applied to the end, one of the metals will expand faster than the other and the coil can operate a switch or valve just as the thermocouple does.
The Recording Thermometer
When a bimetallic coil strip is attached to a long arm lever, with a marker at the end and a drum that has graph paper, a recording thermometer can be made. This instrument works much the same as a seismograph.
The Infrared Thermogram
If an object is warmer than absolute zero it gives off infrared radiation (IR). The infrared radiation can be photographed with special films or detected by special sensors that display colored images. The brightness or color of the image indicates the temperature of the object.
Topic Review p. 201