Judene Zlade, age 9, of Visalia, Calif, for the question:
Thermos, with a tight stopper keeps hot liquids hot and cold liquids cold. It is also called a vacuum flasks which gives us a clue as to how it works. The heat or cold is sealed inside by a thin laser of vacuum. This vacuum is a blanket of almost empty space. The outer metal thermos bottle encloses a silvery glass flask. This flask is really two flasks, one inside the other. The air, or most of it, is removed from the space between them and their rims are sealed together.
The hot or cold liquid is placed inside this double flask. The vacuum between the two walls does the work of keeping the heat in or out. For heat cannot pass through a vacuum. It needs help to travel from place to place. There are three ways in which heat can move from one object to another. And the vacuum flask puts a stop to two and part of the third of these ways.
An object becomes hot when its molecules gain speed. Everything is made of molecules which, in turn, are made of atoms. At ordinary temperatures these molecules are in motion. A substance has to become very, very cold indeed for its molecules to become quite still. The warmer an object is, the faster its molecules are moving.
Heats as we know, tends to spread from one object to another. A silver spoon becomes hot in boiling water. A hot radiator warms the air of a room. The sun heats the earth. These represent the three methods whereby heat travels from place to place.
The boiling water heats the spoon by conduction. The speeding molecules of the water strike the molecules on the surface of the spoon. These surface molecules speed up and crash into those behind them. The heat energy is conducted through the whole spoon.
A radiator heats the room by convection. It heats the air which touches it. This warmed air expands and rises. Soon a current of warm air is circulated through the room.
The sun warms the earth by radiation. Powerful electromagnetic waves carry the heat energy. This energy is absorbed by land and sea. Molecules are speeded up and the earth becomes warm. A vacuum flask is planned to check these three methods by which heat moves from place to place. The vacuum space sealed in the double flask prevents convection. There is no air to warm up and circulate as it does around a radiator. The heat can escape from a hot liquid inside the double flask. Hot soup stays hot. No heat can cross the vacuum barrier to warm a cold drink inside the flask. Cold milk stays cold.
The silvery walls of a vacuum are designed to stop heat by radiation. Those shiny surfaces reflect off heat and light rays. So heat cannot get in or out of the flask either by convection or by radiation.
But it still has another way to travel ‑ by conduction. Some heat by conduction does travel down the inner neck of the flask. But some substances conduct better than others h silver spoon gets hotter in hot water than a glass rod. Silver is a good conductor of heat. Glass is a poor conductor. And the inner vacuum flask is made of glass. The escape of heat, then, is very slow and we can expect the liquid to stay hot or cold for many hours.