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so the liquid cannot normally return to the bulb. Th us, the F C K
Boiling
thermometer shows the highest reading, even if the tempera-
point of 212°F 100°C 373 K
ture it measures has fluctuated up and down during the read- water

ing. The liquid must be forced back into the bulb by a small

swinging motion, bulb end down, then sharply stopping the
swing with a snap of the wrist. The inertia of the mercury in

the bore forces it past the constriction and into the bulb. Th e Freezing
fever thermometer is then ready to use again. point of 32°F 0°C 273 K
water
Today, scientists have developed a different type of ther-

0°F –18°C 255 K
mometer and a way around the problems of using a glass
mercury fever thermometer. This new approach measures the

Absolute
internal core temperature by quickly reading infrared radia- zero –459°F –273°C 0 K
tion from the eardrum. All bodies with a temperature above
absolute zero emit radiation, including your body (see radia- FIGURE 4.7 The Fahrenheit, Celsius, and Kelvin temperature
scales.
tion on p. 98 ). The intensity of the radiation is a sensitive func-

tion of body temperature, so reading the radiation emitted will
tell you about the temperature of that body.

The human eardrum is close to the hypothalamus, the (1686–1736) in about 1715. Fahrenheit invented a mercury-
body’s thermostat, so a temperature reading taken here must be in-glass thermometer with a scale based on two arbitrarily

close to the temperature of the internal core. You cannot use a chosen reference points. The original Fahrenheit scale was
mercury thermometer in the ear because of the very real danger based on the temperature of an ice and salt mixture for the
of puncturing the eardrum, along with obtaining doubtful read- lower reference point (0°) and the temperature of the human

ings from a mercury bulb. You can use a device to measure the body as the upper reference point (about 100°). Thus, the orig-
infrared radiation coming from the entrance to the ear canal, inal Fahrenheit scale was a centigrade scale with 100 divisions
however, to quickly obtain a temperature reading. A micro- between the high and the low reference points. Th e distance
processor chip is programmed with the relationship between between the two reference points was then divided into equal

the body temperature and the infrared radiation emitted. Using intervals called degrees. There were problems with identifying
this information, it calculates the temperature by measuring the a “normal” human body temperature as a reference point,
infrared radiation. The microprocessor then sends the tempera- since body temperature naturally changes during a given day

ture reading to an LCD display on the outside of the device, and from day to day. Some people “normally” have a higher
where it can be read almost immediately. body temperature than others. Some may have a normal body
temperature of 99.1°F, while others have a temperature of 97°F.
The average for a large population is 98.6°F. The only consis-

CONCEPTS Applied tent thing about the human body temperature is constant
change. The standards for the Fahrenheit scale were eventually

changed to something more consistent, the freezing point and
Human Thermometer?
the boiling point of water at normal atmospheric pressure. Th e
Here is a way to find out how well the human body senses original scale was retained with the new reference points,
temperature. Obtain three containers that are large enough however, so the “odd” numbers of 32°F (freezing point of
to submerge your hand in water. In one container, place water) and 212°F (boiling point of water under normal pres-
enough ice water, including ice cubes, to cover a hand.

In a second container, place enough water as hot as you sure) came to be the reference points. There are 180 equal inter-
can tolerate (without burning yourself) to cover a hand. Fill vals, or degrees, between the freezing and boiling points on
the third container with enough moderately warm water to the Fahrenheit scale.
cover a hand. Th e Celsius scale was invented by Anders C. Celsius
Submerge your right hand in the hot water and your (1701–1744), a Swedish astronomer, in about 1735. Th e Cel-
left hand in the ice water for one minute. Dry your hands sius scale uses the freezing point and the boiling point of
quickly, then submerge both in the warm water. How does water at normal atmo spheric pressure, but it has diff erent
the water feel to your right hand? How does it feel to your arbitrarily assigned values. The Celsius scale identifi es the

left hand? How well do your hands sense temperature? freezing point of water as 0°C and the boiling point as 100°C.

There are 100 equal intervals, or degrees, between these two
reference points, so the Celsius scale is sometimes called the
centigrade scale.
TEMPERATURE SCALES There is nothing special about either the Celsius scale or

There are several referent scales used to define numerical values the Fahrenheit scale. Both have arbitrarily assigned numbers,

for measuring temperatures (Figure 4.7). Th e Fahrenheit scale and neither is more accurate than the other. The Celsius scale

was developed by the German physicist Gabriel D. Fahrenheit is more convenient because it is a decimal scale and because

90 CHAPTER 4 Heat and Temperature 4-6