Lists of good and poor conductors may then be made, the teacher adding to the list. Good: metals; poor: wood, horn, bone, cloth, leather, air, water, hair, asbestos, ashes, rock, earth.
PROBLEMS
1. If the interior of the earth is very hot, why do we not feel it?
2. How can the cold snow keep the earth warm?
3. Why does your hand freeze to metals but not to wood?
4. Let the children try to find other instances: wools or furs for clothing, fur coats on northern animals, feathers on birds, down quilts, tea cosies, sawdust for packing ice, double windows, wooden handles for hot irons, asbestos coating for steam pipes.
The Miners' Safety-Lamp: This is a most important application of conduction. Get from the tinsmith a piece of brass gauze six inches square. Raise the wick of the spirit-lamp causing it to give a high flame and bring the gauze down upon the flame till it touches the wick. Note that the flame does not rise above the gauze. Hold a piece of paper above the gauze near the flame and note that it does not take fire. Note also that the gauze soon becomes hot. The brass wires conduct the heat of the flame rapidly away so that there is not heat enough above the gauze to cause combustion. Now roll the gauze into a hollow cylinder, pin the edges together, insert a cork at each end, and have a short candle fastened to the lower one. Try to light the candle with the lamp through the gauze. It is not easily done.
The miner carries a lamp made like this, so that if he should be in the presence of the explosive gas, "fire damp", it would not explode because of the wire gauze shield.
CONVECTION
Water is not a conductor, how then is it heated?