Question

The higher and lower fixed points on a thermometer are separated by 160 mm. When the length of the mercury thread above the lower point is 40 mm, the temperature reading would be :

Options

• 60°C

• 100°C

• 40°C

• 25°C

Answer 1

25°C

Explanation - 

Let the thermometer's upper fixed point (UFP) be 160 mm.

Set the thermometer's lower fixed point (LFP) to 0 mm to keep the separation distance at 160 mm. 

Fixed point of thermometer, P = 40 nm

`(("P" - "LFP")/("UFP" - "LFP")) = "C"/100`

⇒ `(40 - 0)/(160 - 0) = "C"/100 ⇒ 1/4 = "C"/100 => "C" = 25°C`