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why doctor dips the thermometer in a liquid

There are a few different types of thermometers and a few different ways body temperature can be taken:. - A tympanic thermometer measures the body temperature by taking a reading from the tympanic membrane in the ear. - The old mercury thermometers aren't used anymore, the risk of them breaking is too high. - Digital thermometers are about 15cm long and can measure oral, axillary or rectal temperature.

They have plastic coverings to go over the top. Even though it is rarely used (except in paediatrics), for obvious reasons, the rectal method of measuring body temperature is the most accurate.
When a mercury thermometer is immersed in a hot liquid, the mercury level goes down at first and then goes up. Why? There are two physical properties that affect the thermometer as it comes into thermal equilibrium with the body in which the temperature is to be measured.

They are the thermal expansion and the thermal conductivity of the materials that compose the thermometer. Glass has a relatively low thermal conductivity, approximately 1 W/(m- C), and a low volume expansion coefficient, approximately 10 ( /C), a factor of 10 greater than glass.

As energy flows into the thermometer the glass warms first and starts to expand and its volume increases. Because of the low thermal conductivity of the glass the mercury does not expand initially, so its volume doesnomt change initially and the height of the mercury column actually decreases due to the larger capillary volume of the glass.

Eventually the glass comes into thermal equilibrium and ceases to expand while the mercury continues to expand until it too comes into thermal equilibrium with both the glass and the body. The rate of the expansion of the mercury is determined by in part by the thermal conductivity of the glass. Answered by: Steve Beecher, Ph. D. , Physics Professor, Nazareth College of Rochester

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