Why does a feather comforter keep you so warm?
When you heat one end of a nail, how does the heat get through
to the other end?
How does a stove make the whole room warm?
Here is a way to make heat run a race. See whether the heat that goes
through an iron rod will beat the heat that goes through a glass rod,
or the other way round:
[Illustration: FIG. 56. The metal balls are fastened to the iron and
glass rods with drops of wax.]
EXPERIMENT 41. Take a solid glass rod and a solid iron rod,
each about a quarter inch in diameter and about 6 inches long.
With sealing wax or candle grease stick three ball bearings or
pieces of lead, all the same size, to each rod, about an inch
apart, beginning 2 inches from the end. Hold the rods side by
side with their ends in a flame, and watch the balls fall off
as the heat comes along through the rods. The heat that first
melts off the balls beats.
[Illustration: FIG. 57. Does the heat travel faster through the iron
or through the glass?]
What really happens down among the molecules when the heat travels
along the rods is that the molecules near the flame are made to move
more quickly; they joggle their neighbors and make them move faster;
these joggle the ones next to them, and so on down the line. Heat
that travels through things in this way is called _conducted_ heat.
Anything like iron, that lets the heat travel through it quickly, is
called a _good conductor_ of heat. Anything like glass, that allows
the heat to travel through it only with difficulty, is called a _poor
conductor_ of heat, or an _insulator_ of heat.
A silver spoon used for stirring anything that is cooking gets so
hot all the way up the handle that you can hardly hold it, while the
handle of a wooden spoon never gets hot. Pancake turners usually have
wooden handles. Metals are good conductors of heat; wood is a poor
conductor.
An even more obvious example of the conducting of heat is seen in a
stove lid; your fire is under it, yet the top gets so hot that you can
cook on it.
When anything feels hot to the touch, it is because heat is being
conducted to and through your skin to the sensitive little nerve ends
just inside. But when anything feels cold, it is because heat is being
conducted away from your skin into the cold object.
AIR CARRIES HEAT BY CONVECTION. One of the poorest conductors of heat
is air; that is, one particle of air can hardly give any of its heat
to the next particle. But particles of air move around very easily and
carry their heat with them; and they can give the heat they carry with
them to any solid thing they bump into. So when air can move around,
the part that is next to the stove, for instance, becomes hot; this
hot air is pushed up and away by cold air, and carries its heat with
it. When it comes over to you in another part of the room, some of its
heat is conducted to your body. When air currents--or water currents,
which work the same way--carry heat from one place to another like
this, we say that the heat has traveled by _convection_.
[Illustration: FIG. 58. Convection currents carrying the heat of the
stove about the room.]
Since heat is so often carried to us by convection,--by warm winds,
warm air from the stove, warm ocean currents, etc.,--it _seems_ as if
air must be a good conductor of heat. But if you shut the air up into
many tiny compartments, as a bird's feathers do, or as the hair on an
animal's back does, so that it cannot circulate, the passage of heat
is almost completely stopped. When you use a towel or napkin to lift
something hot, it is not so much the fibers of cotton which keep the
heat from your hand; it is principally the very small pockets of air
between the threads and even between the fibers of the threads.
COLD THE ABSENCE OF HEAT. Cold is merely the absence of heat; so if
you keep the heat from escaping from anything warm, it cannot become
cold; while if you keep the heat from reaching a cold thing it cannot
become warm. A blanket is just as good for keeping ice from melting,
by shutting the heat out, as it is for keeping you warm, by holding
heat in.
_APPLICATION 32._ Explain why ice is packed in straw or
sawdust; why a sweater keeps you warm.
Select from the following list the good conductors of heat
from the poor conductors (insulators): glass, silver, iron,
wood, straw, excelsior, copper, asbestos, steel, nickel,
cloth, leather.
[Illustration: FIG. 59. Diagram of a hot-water heater. What makes the
water circulate?]
INFERENCE EXERCISE
Explain the following:
171. If the axle of a wheel is not greased, it swells until it
sticks fast in the hub; this is a hot box.
172. When you have put liquid shoe polish on your shoes, your
feet become cold as it dries.
173. The part of an ice-cream freezer which holds the cream
is usually made of metal, while that which goes outside and
contains the ice and salt is usually made of wood.
174. The steam in a steam radiator rises from a boiler in the
basement to the upper floors.
175. When you throw a ball, it keeps going for a while after
it leaves your hand.
176. Clothes keep you warm, especially woolen clothes.
177. The Leaning Tower of Pisa does not fall over.
178. It is almost impossible to climb a greased pole.
179. Heat goes up a poker that is held in a fire.
180. A child can make a bicycle go rapidly without making his
feet go any faster than if he were walking.
CHAPTER FIVE
RADIANT HEAT AND LIGHT