Chapter 7-PRINCIPLES OF MEASUREMENT 



back to the same point in the year 1900. 

 By definition, 1 second was equal to 

 1/31,556,925.9747 of the tropical year 1900. 

 The subdivision of the tropical year 1900 into 

 smaller time intervals was accomplished by 

 means of laboratory-type pendulum clocks, 

 together with observation of natural phenomena 

 such as the nightly movement of the stars and 

 the moon. 



Although the standard of time was changed 

 to the tropical year 1900 in 1960, the same 

 General Conference of Weights and Measures 

 that approved this change also urged that work 

 go forward on the development of an atomic 

 clock. In 1967, the General Conference of Weights 

 and Measures adopted as the basic standard 

 of time the time required for the transition 

 between two energy states of the cesium- 133 

 atom. In accordance with this standard, 1 second 

 is defined as 9,192,631,770 cycles of this parti- 

 cular transition in the cesium- 133 atom. 



The United States standard of time is main- 

 tained by a cesium clock which is kept at the 

 National Bureau of Standards laboratories in 

 Boulder, Colorado. The time signals that are 

 broadcast by four radio stations operated by 

 the National Bureau of Standards are based on 

 this cesium clock. 



MEASUREMENT OF TEMPERATURE 



Temperature is measured by bringing a 

 measuring system (such as a thermometer) into 

 contact with the system in which we need to 

 measure the temperature. We then measure 

 some property of the measuring system--the 

 expansion of a liquid, the pressure of a gas, 

 electromotive force, electrical resistance, or 

 some other mechanical, electrical, or optical 

 property that has a definite and known relation- 

 ship with temperature. Thus we infer the tem- 

 perature of the measured system by the mea- 

 surement of some property of the measuring 

 system. 



But the measurement of a property other 

 than temperature will take us only so far in 

 utilizing the measurement of temperature. For 

 convenience in comparing temperatures and in 

 noting changes in temperature, we must be able 

 to assign a numerical value to any given tem- 

 perature. For this we need temperature scales. 



Until 1954, temperature scales were con- 

 structed around the boiling point and the freezing 

 point of pure water at atmospheric pressure. 

 These two fixed and reproducible points were 



used to define a fairly large temperature in- 

 terval which was then subdivided into the uni- 

 form smaller intervals called d egrees. The two 

 most famiUar temperature scales constructed 

 in this manner are the Celsius scale and the 

 Fahrenheit scale. 



The Celsius scale is often called the centi- 

 grade scale ii the United States and Great 

 Britain. By international agreement, however, 

 the name was changed Trom centigrade to Cel- 

 sius in honor of the eighteenth-century Swedish 

 astronomer, Anders Celsius. The symbol for 

 a degree on this scale (no matter whether it is 

 called Celsius or centigrade) is °C. The Cel- 

 sius scale takes 0° C as the freezing point and 

 100" C as the boiling point of pure water at 

 atmospheric pressure. The Fahrenheit scale 

 takes 32° F as the freezing point and 212° F 

 as the boiling point of pure water at atmospheric 

 pressure. The interval between freezing point 

 and boiling point is divided into 100 degrees 

 on the Celsius scale and divided into 180 degrees 

 on the Fahrenheit scale. 



Since the actual value of the interval between 

 freezing point and boiling point is identical, it 

 is apparent that numerical readings on Celsius 

 and Fahrenheit thermometers have no absolute 

 significance and that the size of the degree is 

 arbitrarily chosen for each scale. The relation- 

 ship between degrees Celsius and degrees 

 Fahrenheit is given by the formulas 



°F = -^ °C f 32 

 5 



°C = -^ (°F - 32) 

 y 



Many people have trouble remembering these 

 formulas, with the result that they either get 

 them mixed up or have to look them up in a 

 book every time a conversion is necessary. If 

 you concentrate on trying to remember the basic 

 relationships given by these formulas, you may 

 find it easier to make conversions. The essen- 

 tial points to remember are these: 



1. Celsius degrees are larger than Fahren- 

 heit degrees. One Celsius degree is equal to 1.8 

 Fahrenheit degrees, and each Fahrenheit degree 

 is only 5/9 of a Celsius degree. 



2. The zero point on the Celsius scale re- 

 presents exactly the same temperature as the 

 32-degree point on the Fahrenheit scale. 



127 



