7. Directions. (A) Heat the bent portion of the needle in the candle flame ([Exp. 2]) until it is red-hot, then immediately plunge the needle into the water.
(B) Test its brittleness and hardness, as in [Exp. 2].
8. Hardening; Tempering. Good steel is a very valuable material; the same piece may be made hard or soft at will. By sudden cooling, the steel becomes very hard. This process is called hardening, but it makes the steel too brittle for many purposes. By tempering is meant the "letting down" of the steel from the very hard state to any desired degree of hardness. This may be done by suddenly cooling the steel when at the right temperature, it not being hot enough to produce extreme hardness. (The approximate temperature of hot steel can be told by the colors which form on a clean surface. These are due to oxides which form as the steel gradually rises in temperature.)
EXPERIMENT 4. To test the hardening properties of soft iron.
Apparatus. A piece of soft iron wire about 3 in. (7.5 cm.) long (No. 4); the candle, water, etc., of [Exp. 3].
9. Directions. (A) Test the wire by bending and filing.
(B) Heat the wire in the candle flame as you did the needle ([Fig. 1]), then cool it suddenly with the water. Study the results.
10. Discussion. Soft iron contains much less carbon than steel. The hardening quality which steel has is due to the proper amount of carbon in it. If you have performed the experiments so far, you will be much more able to understand later ones, and you will see why we are obliged to use soft iron for some parts of electrical apparatus, and hard steel for other parts.