MEASUREMENT OF HEAT AND TEMPERATURE 



127 



moving. As a body is cooled the molecules move more 

 slowly. 



Heat also exists as a form of radiant energy. Hot 

 bodies send out waves of heat by radiation. These 

 waves travel very swiftly through a vacuum, that is, 

 through space without matter in it. In order to explain 

 this phenomenon scientists have imagined that all 

 space, even that between molecules of matter, is filled 

 with some weightless, elastic fluid which is called 

 ether. It is imagined further that the rapidly moving 

 molecules of a hot body disturb this ether. This dis- 

 turbance passes through the ether, in the form of a 

 wave, at the tremendous speed of 186,264 miles per 

 second. Thus radiant heat energy may be defined as 

 a wave motion in ether. 



Heat and temperature are not the same. Heat is an 

 invisible form of energy; hence we must study it 

 from the effects which it produces on various sub- 

 stances. In everyday life and in science we commonly 

 measure the temperature of materials and speak of 

 them as hot or cold. It is important to know, how- 

 ever, that temperature is a measurement of intensity 

 of heat and not quantity of heat. A cup full of water 

 may be at the same temperature as a barrel full of 

 water, but the barrel of water will contain much more 

 heat. This fact may be stated in another way: the 

 water in the cup and the water in the barrel have the 

 same degree of intensity of heat; but the barrel of 

 water contains a greater quantity of heat because the 

 total energy of moving molecules is greater, as there 

 are many more of them in the barrel. Also a cup of 

 boiling water would contain less heat than a barrel 

 of water at a lower temperature, even though the tem- 

 perature or intensity of heat in the cup is greater. 

 To measure heat or temperature, then, consists of 

 measuring two different aspects of heat energy. 



How is temperature measured? Temperature is 

 measured by means of an instrument called a ther- 

 mometer. The first thermometer was made by Gali- 

 leo, a great Italian scientist, in 1592. It consisted of 

 a bulb of air (see Fig. 197) connected with a glass 

 tube, the end of which dipped into a container of 

 water. This type of thermometer did not prove very 

 satisfactory because the air inside it was affected by 

 changes in air pressure as well as by changes in 

 temperature. A few years later Galileo made a differ- 

 ent thermometer by placing alcohol in a sealed tube. 



Before the time of Galileo temperature could be 

 judged only through the sense of touch. This, as you 

 know from experience, is extremely unreliable. On 

 entering a house on a cold day we may think the 

 house is very warm. After being in the house a while 

 we may then think it is cool, whereas the temperature 

 indoors may not have changed a single degree. To 

 be scientific about temperature changes we now make 



Fisher Scientific Co. 

 FIG. 197. GALILEO'S 

 AIR THERMOMETER 



accurate measurements with carefully constructed 

 thermometers. 



How are thermometers con- 

 structed? As both centigrade and 

 Fahrenheit thermometers are in use 

 in America, it is well to under- 

 stand both. They are glass tubes 

 containing mercury or colored alco- 

 hol. Above the liquid in the tube is 

 a vacuum. When the temperature 

 rises the liquid expands and the ex- 

 pansion is measured by the change 

 in length. When the temperature 

 falls the liquid contracts. 



Modern thermometers are 

 marked off in degrees in the follow- 

 ing manner: the bulb of the ther- 

 mometer is placed in melting ice, 

 and the point at which the mercury or alcohol in the 

 tube stops is called the freezing point. Then the bulb is 

 placed in steam escaping from boiling water, and the 

 point at which the mercury or alcohol stops is marked 



the boiling point. The differ- 

 ence between the Fahren- 

 heit and centigrade ther- 

 mometers is that different 

 numbers are used to ex- 

 press the degrees of tem- 

 perature represented by the 

 boiling point and freezing 

 point of water. On the Fah- 

 renheit scale the boiling 

 point of water is 212 and 

 the freezing point of water 

 is 32. On the centigrade 

 scale the boiling point of 

 water is 100 and the freez- 

 ing point of water is 0. 

 Thus 100 degrees on the 

 centigrade scale equal 180 

 on the Fahrenheit scale, or in other words one degree 



180 



TIC. 198. EARLY FLORENTINE 

 THERMOMETERS 



centigrade equals 



100 



9 

 or - of 



a Fahrenheit degree. 



Study Figure 200 carefully ; learn how to compare the 

 two scales and how to transpose one to the other. 



The centigrade thermometer is the more conven- 

 ient to use and is preferred by scientists. For measur- 

 ing temperature changes in public places the Fahren- 

 heit thermometer is still widely used in this country. 



Heat affects substances in different ways. When a 

 substance is heated, that is, when heat energy is added 

 to it, the results may be different. For example, when 

 ice is heated it melts, but its temperature does not 

 change until all of the ice has turned to a liquid. On 



