Why Mercury is used in Thermometers?? Why not Water?
Ever ruminated why only mercury is used in thermometers or in blood pressure measuring devices?
Well, when one is asked for the reason, a lot of people simply don't know and don't care, but the others who answered their answers were surprising, these answers included, -
- " Mercury is used in thermometers since its the only metal at liquid state in room temp." Now so what? the answer followed "so something must be happening".
Well is it necessary to use a metal for this purpose?
NO. Not at all. there is nothing like that. Metallic characteristics have nothing to do with measuring temperatures. This is something a lot of people don't know. Even water can be used in place of mercury but there is a major disadvantage because of which it is not used.
Why Mercury?
Its because of its density. Mercury has a density 13.6 gram/cm^3, Which enables us to measure wide and most favourable range of pressure in a small volume. It is the density of mercury that make it a perfect choice for thermometers and bp machines. 1 atm is equivalent to 760mm of Hg (Mercury).
{Provided the cross sectional area is very small}
What If Water Is Used In Place Of Mercury?
Well, this is the question one must be thinking about now?
If water is used then the length of the device containing it will be too large.
So, to better understand this, let's see the answer to this question-
- If water is used in place of mercury, then what should be the minimum length of barometer tube to measure the normal atmospheric temperature?
{Barometer - an instrument measuring atmospheric pressure, used especially in forecasting the weather and determining altitude.}
**-- Thermometer too uses the same concept based on temperature difference to show the variations. --**
BAROMETER
We know 1 atm = 76 cm of Hg.
let p1, p2 be density of water and mercury respectively and h1, h2 be their heights.
Now,
1 atm = p1 x g x h1 = p2 x g x h2
{p1 = 1g/cm^3 and p2 = 13.6g/cm^3}
thus,
1 x h1 = 13.6 x 76
h = 1033 cm
h = 10.33 m
Clearly,
what could be represented by 76 cm of Hg requires 10.33 m of water.
Similarly in case of thermometer too, using water would be impractical because of its high length requirement.
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