Explain the following:
461. If a pin is put through a lamp cord, a fuse is likely to blow out.
462. The wall paper back of a picture is often darker than that on the rest of the wall.
463. If you wet an eraser, it rubs through the paper.
464. Clothes are hot after being ironed.
465. If you drop candle grease on your clothes, you can remove the grease by placing a blotter over it and pressing the blotter with a warm iron.
466. Milliners cover hats that are on display in windows where the sun shines in on the hats.
467. You pull down on a rope when you try to climb it.
468. In taking a picture, you expose the sensitive film or plate to the light for a short time.
469. Good cameras have an adjustable front part so that the lens may be moved nearer to the plate or film, or farther from it, according to the distance of the object to be photographed.
470. A pencil has to be resharpened frequently when it is much used.
Section 50. Chemical change caused by electricity.
How are storage batteries charged?
How is silver plating done by electricity?
You have already done an experiment showing that electricity can start chemical change, for you changed water into hydrogen and oxygen by passing a current of electricity through the water.
The plating of metals is made possible by the fact that electricity helps chemical change. You can nickel plate a piece of copper in the following manner:
Experiment 103. Dissolve a few green crystals of "double nickel salts" in water, until the water is a clear green. The water should be about 2 or 3 inches deep in a glass or china bowl that is not less than 5 inches across.
Lay two bare copper wires across the bowl, about 3 inches apart, as shown in Figure 177. Connect the positive wire from a storage battery, or the wire from the carbon of a battery of three or four cells, to an end of one bare wire. Connect the negative wire from the storage or the negative wire from the zinc of the other battery to an end of the second bare wire.
Fig. 176. The copper and the nickel cube ready to hang in the cleansing solution.
Now fasten a fine bare wire 5 or 6 inches long around a small piece of copper, and another like it around a piece of nickel, as shown in Figure 176. Then put the piece of copper in the bottom of an evaporating dish, with the wire hanging out, as in Figure 177.
Fig. 177. Cleaning the copper in acids.
Pour over the piece of copper enough of the cleansing solution to cover it.[9] The cleansing solution contains strong acids. If you get any on your skin or clothes, wash it off immediately with ammonia or soda. As soon as the copper is bright and clean, take it out of the cleansing solution and suspend it by the negative wire in the green nickel solution. You can tell if you have it on the negative wire, for in that case bubbles will rise from it during the experiment. The copper should be entirely covered by the nickel solution, but should not touch the bottom or sides of the bowl. Pour the cleansing solution from the evaporating dish back into the bottle. Suspend the nickel, in the same way as the copper, from the positive wire crossing the bowl. When set up, the apparatus should appear as shown in Figure 178.