英文科學(xué)讀本 第四冊(cè)·Lesson 55 Thermometers

Lesson 55 Thermometers

I want you to think of our lesson on the thermometer, said Mr. Wilson. "We followed the various steps of making, testing, filling, and sealing the tubes. Now let us see what we have got. We have an instrument which will tell us that one body is hotter than another. That is all. We want something more than this. We want to say how much hotter it is. We want to show the steps or degrees of heat.

To do this the bulb is first placed in a vessel of melting ice. After it has stood there for a short time, the mercury contracts and sinks to a certain point, but it will not sink lower. This point is marked in some way on the tube, and it shows the melting-point of ice, or, as we more frequently call it, the freezing-point of water. Both mean the same thing. The instrument is next suspended in a vessel, in which water is being boiled, so that it is surrounded on all sides by the steam of the boiling water. The mercury expands with the heat, and rises in the tube to a certain point, but after reaching that point, it will not rise higher, however long it is kept there. This point is marked on the tube as the boiling-point of water. We have now found two very important points—one, that at which water freezes; the other, that at which water boils. Between these two points we may make any steps we please.

All thermometers are made in one way up to this stage of the process, but there are different methods of dividing the space between the freezing and boiling points. In one thermometer the space is divided into 100 equal steps; the freezing-point is marked 0, and the boiling-point 100. This is a very simple arrangement, and the instrument is known as the centigrade thermometer—from the Latin centum, 'a hundred,' and gradus, 'a step.' There are 100 steps or grades between the freezing-point and the boiling-point. This thermometer is not commonly used in our country. The one mostly in use places the freezing-point at 32 degrees, and the boiling-point at 212 degrees, thus making 180 steps or degrees between the two points. We call this instrument the Fahrenheit thermometer, from the name of the man who invented it, and we write the steps 32° or 212° (32 degrees or 212 degrees).

This freezing-point of water is not the lowest temperature that can be reached. Fahrenheit found, by making a mixture of snow and salt, a temperature 32° lower than the freezing-point, and he thought he had actually reached the extreme limit. He called this point 0° or zero, and worked upwards from it. Lower temperatures, however, have been found since his time. Ten degrees below zero on the Fahrenheit thermometer means 32° + 10° = 42° below the freezing-point. In the centigrade thermometer 10 grades below zero signifies 10 grades below the actual freezing-point, because the freezing-point in that instrument is zero."