11.2 Measurement of Temperature and Its Units 309
understood by all. In other words, we need to establish and use the same units and scales that
are understood by everyone.
Another example of how people relied on their senses to quantify temperature is the way
blacksmiths used to use their eyes to estimate how hot a fire was. They judged the temperature
by the color of the burning fuel before they placed the horseshoe or an iron piece in the fire. In
fact, this relationship between the color of heated iron and its actual temperature has been mea-
sured and established. Table 11.1 shows this relationship.
From these examples, you see that our senses are useful in judging how cold or how hot
something is, but they are limited in accuracy and cannot quantify a value for a temperature.
Thus, we need a measuring device that can provide information about the temperature of some-
thing more accurately and effectively.
This need led to the development of thermometers, which are based on thermal expan-
sion or contraction of a fluid, such as alcohol, or a liquid metal, such as mercury. All of you
know that almost everything will expand and its length increase when you increase its tem-
perature, and it will contract and its length decrease when you decrease its temperature. We
will discuss thermal expansion of material in more detail later in this chapter. But for now,
remember that a mercury thermometer is a temperature sensor that works on the principle
of expansion or contraction of mercury when its temperature is changed. Most of you have
seen a thermometer, a graduated glass rod that is filled with mercury. On the Celsius scale,
under standard atmospheric conditions, the value zero wasarbitrarilyassigned to the tem-
perature at which water freezes, and the value of 100 was assigned to the temperature at
which water boils. This procedure is calledcalibrationof an instrument and is depicted in
Figure 11.4. It is important for you to understand that the numbers were assigned arbitrar-
ily; had someone decided to assign a value of 100 to the ice water temperature and a value
of 1000 to boiling water, we would have had a very different type of temperature scale today!
In fact, on a Fahrenheit temperature scale, under standard atmospheric conditions, the tem-
perature at which water freezes is assigned a value of 32, and the temperature at which theTABLE 11.1 The Relation of Color to Temperature of Iron
Color Temperature (F)
Dark blood red, black red 990
Dark red, blood red, low red 1050
Dark cherry red 1175
Medium cherry red 1250
Cherry, full red 1375
Light cherry, light red 1550
Orange 1650
Light orange 1725
Yellow 1825
Light yellow 1975
White 2200Source:MARK’S STANDARD HANDBOOK FOR MECHANICAL ENGINEERS. 8TH EDITION by Baumeister
et al. Copyright 1978 by MCGRAW-HILL COMPANIES, INC. - BOOKS. Reproduced with permission of
MCGRAW-HILL COMPANIES, INC. - BOOKS in the format Textbook via Copyright Clearance Center.Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀