What it shows
Our skin is not equally sensitive everywhere. The smallest gap at which a person feels two points instead of one shows where touch receptors are packed more closely.
The science
Skin contains touch receptors, and their number per square centimetre differs across the body. The fingertip and lips have many receptors and large areas of brain devoted to them, so two close points are felt as two. On the back or forearm, receptors are fewer and one receptor covers a wider area, so points must be farther apart to be felt as two. The brain, not the skin alone, interprets the signals.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two blunt wooden toothpicks or two bent paper clips with rounded ends
- A ruler with millimetre marks
- A blindfold or a sleeve to block vision
- A partner volunteer
- A notebook with a ready table
- Cardboard strip with pairs of holes 2, 5, 10, 20 and 40 mm apart (optional)
Safety
For this project: Use only blunt points and the lightest touch. Stop if the volunteer feels any pain. An adult must supervise.
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.
Hold the two blunt points at a known spacing, starting at 2 mm, using the ruler to set them.
Ask the volunteer to look away and then touch one or both points lightly and at the same time on the fingertip of the index finger.
Ask whether they feel one point or two, and write the answer in the table.
Mix in some tests with only one point so the volunteer cannot guess the pattern.
Increase the gap in steps (2, 5, 10, 20 and 40 mm) until the volunteer reports two points correctly every time.
Repeat on the palm, the back of the hand, the forearm and the back of the neck.
Record the smallest gap at which two points are always felt as two for each place.
Draw a bar graph of the smallest gap against body part, and write which places are most sensitive.
What you should see
The fingertip can tell two points only a few millimetres apart, while the forearm or the back of the neck needs a gap of several centimetres. The graph rises from fingertip to back.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Which body part gave the smallest gap?
The fingertip, which has the most touch receptors.
2Why include trials with only one point?
To check the volunteer’s answers are not guesses.
3Why is the forearm less sensitive?
It has fewer receptors per area, so points must be farther apart.
4Which part of the body senses touch signals and decides what they mean?
The brain.
Take it further
Test whether the gap gets smaller after a fingertip is held in cool water for one minute, and discuss why temperature may matter.
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
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