333. A banjo player moves his fingers toward the drum end of
the banjo when he plays high notes.

334. When the filament breaks, an electric lamp will no longer
glow.

335. An inverted image is formed by the lens of a camera.

336. A blown-out fuse may be replaced temporarily with a
hairpin or with a copper cent.

337. Sparks fly when a horse's shoe hits a stone.

338. A room requires less artificial light if the wall paper
is light than if it is dark.

339. Phonographs usually have horns, either inside or outside.

340. An electric car needs only one wire to make it go.

SECTION 37. _Resistance._

What makes an electric heater hot?

Why does lightning kill people when it strikes them?

What makes an electric light glow?

We have talked about making electricity work when it flows in a steady
stream, and everybody knows that it makes lights glow, makes toasters
and electric stoves hot, and heats electric irons. But did it
ever strike you as remarkable that the same electricity that flows
harmlessly through the wires in your house without heating them,
suddenly makes the wire in your toaster or the filament in your
incandescent lamp glowing hot? The insulation is not what keeps the
wire cool, as you can see by the next experiment.

EXPERIMENT 69. Between two of the laboratory switches you will
find one piece of wire which has no insulation. Turn on the
electricity and make the lamp glow; see that you are standing
on dry wood and are not touching any pipes or anything
connected to the ground. Feel the bare piece of wire with your
fingers. Why does this not give you a shock? What would happen
if you touched your other hand to the gas pipe or water pipe?
_Do not try it!_ But what would happen if you did?

The reason that the filament of the electric lamp gets white hot
while the copper wire stays cool is this: All substances that conduct
electricity resist the flow somewhat; there is something like friction
between the wire and the electricity passing through it. The smaller
around a wire is, the greater resistance it offers to the passing of
an electric current. The filament of an electric lamp is very fine
and therefore offers considerable resistance. However, if the filament
were made of copper, even as fine as it is, it would take a much
greater flow of electricity to make it white hot, and it would be
very expensive to