Andrew Sanderson, age 15, of Wilmington, Del., for his question:
CAN A PERFECT VACUUM BE MADE?
A vacuum is a space which has no matter in it. Man can create
a partial vacuum but no one has ever created an absolute vacuum. Anabsolute vacuum would be an enclosed space with all the air takenout. In practice, we speak of a vacuum as a space from which most ofthe air or other gas has been taken, and in which the pressure isextremely low. But this is actually only a partial vacuum.In the air we breathe, there are about 300 billion molecules ofgas per cubic inch. The best partial vacuum may cut the gas moleculesdown to only a few million.
Among the most nearly complete vacuums that have been made isone in which the pressure of the gas is about one ten thousand millionthof the normal pressure of the air.Scientists speak of high or low partial vacuums, depending onhow completely the air or any other gas has been taken out of an enclosedspace. Any space in which the pressure is about one thousandth ofthat of the atmosphere is usually called a vacuum.
The important thing about a vacuum is that it is usually a usefulemptiness. For example, drinking a fluid through a straw helps toprove this. We do not pull the liquid up through the straw. Instead,by sucking on one end, we take some of the air out of the straw. The air outside exerts pressure on the liquid and forces it up and outthrough the partial vacuum in the straw. The vacuum cleaner is another example of the usefulness of thistype of a vacuum. The pump used to pump water into our homes, theair pumps which inflate automobile tires and the gas intake of an automobile motor also make use of the same principle as the drinking straw.
Most electronic tubes contain a high vacuum. This keeps the airor gas inside from being affected by the passage of electrons across the tube.
In some vacuum tubes in use today, as much air as possible istaken out of the tubes by means of an air pump. Then a chemical called a "getter" is placed inside the tube. This increases the vacuum by combining with a large part of the remaining air inside the tube. The chemical then coats the inside surface of the vacuum tube. In the sugar refining industry, a vacuum pan is used. This evaporates liquids at a temperature lower than the ordinary boiling point. The vacuum pan is made up of a closed vessel heated by a steam jacket. An air pump holds the steam in the air constantly from the surface of the liquid inside. The liquid boils and evaporates without danger of burning.
An important characteristic of a vacuum is the fact that sound cannot cross a high vacuum. We can prove this by putting an alarm clock inside a bell jar. As the air inside the bell jar is pumped out, the sound of a ringing clock's bell gradually grows less and less until such a high partial vacuum is reached that the ringing bell can be heard no more.