32. To Amalgamate Battery Plates. To keep the zinc plates or rods in cells from being eaten or dissolved when the circuit is opened, they should be amalgamated; that is, they should have a coating of mercury. The local currents (see text-book) aid in rapidly destroying the zinc, unless it is amalgamated. Do not amalgamate copper plates—merely the zinc ones.
33. Place a few drops of mercury in a butter dish. Dip the zinc into the solution of [App. 14], then lay it upon a flat board. This is necessary with thin sheet-zinc, as it becomes very brittle when coated with mercury, and will not stand hard rubbing. If you also dip a very narrow piece of tin into the dilute sulphuric acid, you can use this as a spoon and lift one drop of mercury at a time from the butter dish to the zinc. By tapping the tin upon the zinc, the mercury will leave the tin. Put the mercury only where the zinc will be under the solutions in the cell, then rub the drops around with a small cloth that has been dipped in the acid. The zinc will become very bright and silvery, due to the mercury. Do not get too much mercury on it, just enough to give it a thin coat, as it will make the thin zinc so brittle that it will very easily break. Amalgamate both sides of the zinc.
CHAPTER III.
MISCELLANEOUS APPARATUS AND METHODS OF CONSTRUCTION.
34. For Annealing and Hardening Steel. (See text-book for reasons why some parts of electrical apparatus should be made of hard steel, while other parts should be made of soft iron.)
35. To anneal or soften spring steel so that you can bend it without breaking it, heat it in a candle, gas, or alcohol flame until it is red-hot; allow the steel to cool in the air slowly.