Lesson 49 The Thermometer

“I think you can all tell me how bodies—solids, liquids, and gases—are affected by heat,” said Mr. Wilson.

“Bodies expand with heat, sir, and contract when the heat is taken away,” said Fred.

“Right,” said Mr. Wilson. “I think too you can give me the reason for this.”

“Heat overcomes the force of cohesion, sir. The particles when heated are not held so firmly together, and therefore the body expands.”

“Very good,” said Mr. Wilson. “Now I want you to pay close attention to a little experiment. I have here a small glass phial, filled with colored water, and fitted with a cork and a piece of glass tubing. The water reaches some little distance above the cork. Here is a basin of hot water, Fred shall come to the front and hold the phial in it. What do you see now?”

“The colored water is rising higher in the tube, sir.”

“Quite right,” said Mr. Wilson. “But what does this prove?”

“It proves that the colored water in the phial is expanding; it wants more room, and so it rises in the tube.”

“But why does it expand?”

“The heat of the water in the basin makes it expand, sir.”

“Exactly,” said Mr. Wilson. “Now let us make the water still hotter by pouring into it some boiling water out of the kettle. As soon as that is done you see the colored water in the tube rise higher and higher. The hotter the liquid into which it is plunged, the higher the water rises in the tube.

“Now we will put the phial into this basin of cold water, and watch again. The colored water is now shrinking or contracting; it is falling in the tube. Hence, you see, that by carefully marking the height of the colored water in the tube each time, we could use our phial and tube as an instrument for measuring the temperature of the water. The phial and tube give us a rough-and-ready but true measurement. They make a simple sort of thermometer. Thermometer means heat-measurer, from two Greek words—therme=heat, and metron=a measure. But what is it that makes the phial and tube a heat-measurer?”

“The expansion and contraction of the water inside does the actual measuring, sir.”

“Just so,” said Mr. Wilson. “But other substances besides water expand with heat, and contract when the heat is taken away. Why would not a bar of iron, for instance, make a good thermometer?”

“Solids do not expand sufficiently to be seen and marked,” said Fred.

“Quite right, Fred, and so of course they would not be suitable for thermometers. Then again, air and other gases expand and contract too rapidly to be of any use in this way. We find liquids are the only bodies fitted for the purpose.

“We said just now that the phial of water made a rough-and-rude sort of thermometer. It is rough and rude because it would not suit all purposes. Suppose, for example, I had stood the phial in ice. What would have happened? The water inside the tube would have become ice too in a short time, and as ice it would have been of no further use for measuring.

“Or suppose I put it over the spirit-lamp. It would boil and become changed into steam, or water-gas, and therefore be useless again. The best liquid for a thermometer is mercury.

“It expands and contracts sufficiently to be seen and marked.

“It does not change into the vapor state, till an exceedingly high temperature is reached.

“It does not freeze, except at a very low temperature.”