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Question

State the principles on which liquid thermometer and digital thermometer work.

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Solution

Liquid thermometer

A thermometer has a glass tube sealed at both ends and is partly filled with a liquid like mercury or alcohol. As the temperature around the thermometer's bulb heats up, the liquid rises in the glass tube. When it is hot, the liquid inside the thermometer will expand and rise in the tube.

Ethanol-filled thermometers are used in preference to mercury for meteorological measurements of minimum temperatures and can be used down to −70 °C (-94 °F). The physical limitation of the ability of a thermometer to measure low temperature is the freezing point of the liquid used.


Digital Thermometers

Digital Thermometers are slowly replacing the conventional mercury thermometer due to the ease of taking reading. People often have the misconception that it contains mercury. Digital Thermometers are mercury free. These thermometers contain thermistor inside the tip which is used to measure the temperature. They provide quick and highly accurate results over the body temperature range.

These thermometers are easy to read with LCD display on them. They are equipped with beep alarm & memory function and can record a wide range of temperature. Doctor’s thermometer which are mostly used can read temperature between 94oF and 105oF (35oC and 42oC). It is three in one thermometer as it can record oral, auxiliary and rectal temperatures.

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