NFERENCE EXERCISE

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.

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 Figu