THERMOMETER
Thermometer: A device that is used to measure the temperature of a body is called thermometer. Some substance have property that changes with temperature. Substance that show changes with temperature can be used as a thermometric material. For example, some substance expand on heating, some changes there colors, some changes there electric resistance, etc. Nearly all these substance expand on heating. Liquids also expand on heating and are suitable as thermometric materials. Common thermometer are generally made using some suit able liquid as thermometric material. A thermometric liquid should have the following properties:
- It should be visible.
- it should have uniform thermal expansion.
- It should have a low freezing point.
- It should have a high boiling point.
- It should not wet glass.
- It should be a good conductor of heat.
- It should have a small specific heat capacity.
Liquid-IN-GLASS THERMOMETER
A liquid-in-glass thermometer has a bulb with a long capillary tube of uniform and fine bore such as shown in fig bellow. A dutiable liquid filled in the bulb. When the bulb contacts a hot object, the liquid in it expands and rises in the tube. The glass steam of a thermometer is thick and acts as a cylindrical lens. This makes it easy to see the liquid level in the glass tube.
Mercury freezes at-39 degree Celsius and boils at 357 degree Celsius. It has all the thermometric properties that is listed above. Thus, mercury is one of the most of the thermometric material. Mercury-in-glass thermometer are widely used in laboratories, Clinics and houses to measure temperature in the range from -10 degree Celsius to 150 degree Celsius.
LOWER AND UPPER FIXED POINTS
A thermometer has a scale on its stem. This scale has to fixed points. The lower fixed point is mark to show the position of liquid in the thermometer when it is placed in ice. Similarly, upper fixed point is marked to show the position of liquid in the thermometer when it is placed in the steam at standard pressure above boiling water.
SCALE OF TEMPERATURE
A scale is marked on the thermometer. The temperature of a body is in contact with the thermometer can be read on that scale. Know a days digital thermometer is also used. Three scales of temperature are in common use. These are:
- Celsius scale or centigrade scale
- Fahrenheit scale
- Kelvin scale
On Fahrenheit scale, the interval between lower and upper fixed points is divided into 180 equal parts as shown in fig bellow. The lower fixed point is marked as 32 degree Fahrenheit and the upper fixed point is marked as 212 degree Fahrenheit.
In SI units, the unit of temperature is Kelvin (K) and its scale is called kelvin scale of temperature as shown in fig. The interval between the upper and the lower fixed point is divided into 100 equal parts. Thus a changes in 1 degree Celsius is equal to change of 1 K. The lower fixed point on this scale corresponded to 273 K. The zero on this scale is called absolute zero and is equal to - 273 degree Celsius.
CONVERSION OF TEMPERATURE FROM ONE
SCALE INTO OTHER TEMPERATURE SCALE
FROM CELSIUS TO KELVIN SCALE
The temperature T on Kelvin scale can be obtained by adding 273 in the temperature C on Celsius scale. Thus
T (K) = 273 + C ............................. (1)
FROM KELVIN TO CELSIUS SCALE
The temperature on Celsius scale can be found by subtracting 273 from the temperature in Kelvin scale. Thus
C = T (K) - 273................................. (2)
FROM CELSIUS TO FAHRENHEIT SCALE
Since 100 division on Celsius scale are equal to 180 division on Fahrenheit scale. Therefore, each decision on Celsius scale is equal to 1.8 division on Fahrenheit scale. Moreover, 0 degree Celsius Corresponds to 32 degree Fahrenheit.
F = 1.8 C + 32 ................................... (3)
Here F is the temperature on Fahrenheit scale and C is the temperature on Celsius scale.
FROM FAHRENHEIT TO CELSIUS SCALE
From equation (3), we can find the temperature on Celsius scale from Fahrenheit scale.
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