What it shows
Human hair gets slightly longer when the air is humid and shorter when it is dry. A long hair pulling on a pointer turns this tiny change into a visible movement, which is how real hair hygrometers worked for two centuries.
The science
Hair is made of keratin, a protein that absorbs water molecules from the air. When water moves in, the hair swells and lengthens by about 2 to 3 per cent between very dry and very damp air. The hair is fixed at one end and wrapped round the pivot of a pointer at the other, so even a small length change swings the pointer through an easy-to-read angle.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- One long human hair, washed with soap to remove oil
- A piece of stiff card or a wooden board
- A light pointer cut from a straw or thin card
- A pin to act as a pivot
- A small weight such as a paper clip
- Tape and a protractor
Safety
For this project: The pin point is sharp; press it into the board with an adult watching.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Push the pin through the pointer near one end and into the board so the pointer swings freely.
Tape the top of the hair near the top of the board and wind the lower end once round the pin close to the pointer, then fix it with a tiny drop of glue.
Hang the paper clip from the pointer tip so the hair stays gently stretched at all times.
Hold the board inside a bathroom just after a hot shower, wait five minutes, and mark the pointer position "wet".
Hold it near a fan in a dry room for ten minutes and mark the position "dry".
Divide the space between the two marks into ten equal steps with the protractor.
Read the pointer every morning and evening for a week and write the readings in a table.
Compare the readings with the humidity shown in a weather app to see how close your scale is.
What you should see
The pointer swings towards "wet" on humid or rainy days and towards "dry" on clear afternoons. Readings should follow the trend of the weather app within two or three steps.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is humidity?
The amount of water vapour in the air.
2Why wash the hair with soap first?
Natural oil coats the hair and slows the way it takes up water.
3Why is a long hair better than a short one?
The change in length is a percentage, so a longer hair moves the pointer further.
4Name a modern instrument that measures humidity.
An electronic capacitive humidity sensor, like those in weather stations.
Take it further
Test whether thick hair, thin hair or nylon thread gives the biggest swing.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Earth, Space and Geography Models
Solar system, seasons, eclipses, volcanoes and the water cycle explained
Science projects
Science Projects for Class 6 to 8
Working models matched to middle-school science topics