This is a ‘house hygrometer’. It measures temperature and humidity. Inside it, a very thin piece of paper is attached to a coil. When it absorbs air moisture, the paper expands, which causes the coil to rotate, which moves the humidity needle. What is the relative humidity shown by the house hygrometer here?
7. A ‘house hygrometer’
How a Wet and Dry Bulb Hygrometer Works
One thermometer measures the air temperature. Its bulb remains dry.
°C 50
10 20 30 40
0
–10 –20
What is the temperature of
the dry bulb? What is the temperature of the wet bulb?
The difference in temperature between the two thermometers indicates relative humidity, which is calculated using a special set of tables.
8. A wet and dry bulb hygrometer
The bulb of the other thermometer is kept wet by a cloth cover that dips into a container of water.
Storage of Instruments
Some instruments, such as the barograph, the maximum and minimum thermometer and the hygrometer, are stored in a wooden box called a Stevenson screen.
The Stevenson screen is painted white to reflect the sun.
Gaps in the sides allow air but not direct sunlight to enter the box.
The Stevenson screen allows for more accurate measurements to be recorded.
