Section 20. _conduction of Heat and Convection._

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:

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.

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.

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.

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.