hed
nothing else.

345. If you were on the moon, you would look up at the earth.

346. Toy balloons burst when they go high up where the air is
thin.

347. You have to put on the brakes to stop a car quickly.

348. Telephone wires are strung on glass supporters.

349. If you pour boiling water into a drinking glass,
frequently the glass will crack.

350. An asbestos mat tends to keep food from burning.

[Illustration: FIG. 132. Pencils ready for making an arc light.]

SECTION 38. _The electric arc._

How can electricity set a house on fire?

THIS IS ONE OF THE MOST IMPORTANT SECTIONS IN THE BOOK.

Do you know that you can make an arc light with two ordinary pencils?
The next experiment, which should be done by the class with the help
of the teacher, shows how to do it.

EXPERIMENT 71. Sharpen two pencils. About halfway between the
point and the other end of each pencil cut a notch all the way
around and down to the "lead," or burn a notch down by means
of the glowing resistance wire. What you call the "lead" of
the pencil is really graphite, a form of carbon. The leads of
your two pencils are almost exactly like the carbons used in
arc lights, except, of course, that they are much smaller.
Turn off the electricity both at the snap switch and at the
knife switch. Fasten the bare end of a 2-foot piece of fine
insulated wire (about No. 24) around the center of the lead in
each pencil so that you get a good contact, as shown in Figure
132. Fasten the other bare end of each wire to either side
of the open knife switch so that when this switch is open the
electricity will have to pass down one wire to the lead of one
pencil, from that to the lead of the other pencil, and from
that back through the second wire to the other side of the
knife switch and on around the circuit, as shown in Figure
133. Keep the two pencils apart and off the desk, while some
one turns on the snap switch and the "flush" switch that lets
the electricity through the resistance wire. Now bring the
pencil points together for an instant, immediately drawing
them apart about half an inch. You should get a brilliant
white arc light.

_Caution: Do not look at this brilliant arc for more than a
fraction of a second unless you look through a piece of smoked
or colored glass._

Blow out the fla