Outline. One finger of each hand is acclimatized to hot or cold water and then dipped into lukewarm water.
Prior knowledge. The general idea of receptors sending impulses to the brain
Advance making ready and materials
A provide of hot and cold water
Apparatus - per group
3 jars or beakers large sufficient to accommodate a finger
Thermometer
Experiment
(a) derive three jars or beakers of about the same size. Fill one with cold water (10-15 °C), one with hot water (40-50 °C) and the third with warm water (about 25 °C).
(b) Place the first finger of the left hand in the cold water and the first finger of the right hand in the hot water. Leave both fingers immersed for at least one minute.
(c) After one minute, take off both fingers from the jars and dip them repeatedly but alternately in the warm water for about a second at a time observation the climatic characteristic sensation in each finger.
Discussion
1 What impression did (i) the left finger, (ii) the right finger give about the climatic characteristic of the
warm water?
2 Why should there be any difference in the sensory facts from the two fingers? How could you modify the experiment to test your suggestion?
3 Does the effect mean that the skin of your fingers is incapable of judging whether an object is hot or cold?
4 What does the effect recommend about the way in which the skin responds to temperature
Discussion - answers
1 The finger which has been immersed in cold water will register warmth. The finger previously held in hot water will register coldness.
2 The difference in sensations can be attributed to the difference in the climatic characteristic of the fingers after one minute's immersion. This can be tested by repeating the experiment with the left-hand finger in the hot water and the right-hand finger in the cold water. The sensation on dipping them both into warm water should be the reverse of the first experiment.
An alternative explanation is that the warmth receptors in the hot water become adapted, i.e. After continued immersion they no longer send impulses to the brain. Consequently, on replacement to lukewarm water, there are few impulses sent to the brain from the warmth receptors of this finger, whereas the warmth receptors in the cold finger fire normally.
3 The results recommend that the fingers detect whether they are gaining or losing heat rather than the actual climatic characteristic of an object. Metal objects at room climatic characteristic will feel cold to the touch because heat is conducted away from the fingers, while wooden objects at the same climatic characteristic feel less cold.
4 The results seem to imply that the thermoreceptors reply to turn in climatic characteristic rather than to any particular temperature. In fact, there is a steady dismissal of nerve impulses from cold and warmth receptors at all temperatures within sure limits but increased bursts of impulses occur during sudden changes of temperature.
Biology Experiments for Teachers. Human Senses: Sensitivity to climatic characteristic
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