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Physics Theory : Thermometers

 
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Thermometer is the device used to measure temperature.
The extremely fine tube (narrow bore) of a thermometer is called a capillary.
The boiling point of water (or condensing point of steam) is 100oC or 212oF or 373 K.
The freezing point of water (or melting point of ice) is 0oC or 32oF or 273 K.
These temperatures are typically used in the calibration of thermometers and are known as the fixed points.
The temperature range that is typically marked on a laboratory thermometer is −10oC to 110oC.

 

Thermometric liquids are those that are used in thermometers.
Some examples of such liquids are mercury and alcohol.
The thermometric fluid used by Galileo Galilei in his thermometer was air.

 

Alcohol, which is used as a thermometric liquid, has the following characteristic properties :
1. Alcohol has a very low freezing point of about −112oC and hence is suitable in thermometers to record very low temperatures.
2. Alcohol has a low boiling point of about 78oC and therefore cannot be used to measure high temperatures.
3. Alcohol can be colored brightly (by adding a dye, generally red) and then it is clearly visible through glass.
4. Alcohol expands more than mercury.
5. Alcohol is fairly inexpensive.
6. Alcohol wets glass.
7. Alcohol is not a good conductor of heat.

 

Mercury, which is used as a thermometric liquid, has the following characteristic properties :
1. Mercury has a high boiling point of about 357oC and therefore can be used to measure temperatures as high as 357oC.
2. Mercury has a freezing point of about −39oC and hence is suitable in thermometers to record low temperatures (although not very low temperatures).
3. Mercury is opaque and has a shining silvery color of its own, making it clearly visible in the capillary tube of a thermometer.
4. Mercury needs very little heat to expand and so it can easily measure the temperature of a body without causing a decrease in the body's temperature.
5. Mercury does not stick to the side of the glass capillary tube of a thermometer. Therefore, it allows accurate temperature measurement.
6. Mercury is a good conductor of heat.
7. Mercury is relatively expensive.

 

Clinical Thermometer (or doctor's thermometer) is used for measuring a person's temperature.
The temperature range that is marked on a clinical thermometer is about 35oC to 43oC (or 95oF to 109oF).
The space above the mercury column in the thermometer is originally vacuum (but may contain some mercury vapor later).
After a clinical thermometer has been used to record a patient's temperature, the mercury is prevented from contracting into the glass bulb by the constriction (kink) unless the thermometer is jerked.

 

Pyrometer is the thermometer that is used for the measurement of high temperatures.
Thermometers that measure high temperatures are calibrated using fixed points other than the temperatures of boiling water and melting ice.

 

Six's maximum and minimum thermometer is used for measuring the day's maximum and minimum temperature.
It consists of a U-shaped capillary tube with bulbs at both ends.
The bend of the U-tube is filled with mercury.
One of the bulbs is completely filled with alcohol and the other bulb is partly filled with alcohol.
The space in the bulb partly filled with alcohol is originally vacuum (but may contain alcohol vapor later).
The thermometric liquid is alcohol, whose expansion and contraction allow the change in temperature to be recorded.
Just above the mercury are two dumbbell-shaped steel indices, which are kept in place by tiny steel springs.
The two dumbbell-shaped steel indices are reset to their initial positions just above the mercury by means of a horse-shoe magnet.

 

The maximum temperature in the Six's thermometer is recorded as follows.
Due to a rise in the temperature during the day, the alcohol in the completely-filled bulb expands and exerts a pressure on the mercury.
The mercury moves toward the partly-filled bulb and pushes up the index in the capillary closer to the partly-filled bulb. Thus, the index closer to the partly-filled bulb records the maximum temperature.

 

The minimum temperature in the Six's thermometer is recorded as follows.
Due to a fall in the temperature during the night, the alcohol in the completely-filled bulb contracts.
The mercury moves toward the completely-filled bulb and pushes up the index in the capillary closer to the completely-filled bulb. Thus, the index closer to the completely-filled bulb records the minimum temperature.

 

Physics Quiz on Thermometers.
 
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