Lesson 2-2 Thermometers


A person's body temperature is measured using an instrument called a thermometer. The word "thermometer" comes from the Greek word therme (heat) and the French word metre (measure). There are two general types of thermometers--the glass thermometer and the electric thermometer. The glass thermometer is easier to carry than the electric thermometer and is cheaper to replace. The electric thermometer measures temperatures faster and does not have to be sterilized after each use as does the glass thermometer.


Electronic Forehead  Thermometer

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Digital Thermometer

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A glass thermometer consists of a stem and bulb. The stem (long part) of the thermometer has a hollow shaft running almost the entire length of the stem. The bulb of the thermometer contains a small amount of mercury, a metal that is liquid at ordinary temperatures. The thermometer is designed so that mercury from the bulb can enter the hollow shaft in the stem. If you hold the mercury thermometer so that the bulb is pointed up; however, the mercury will not flow into the shaft. The mercury only enters the shaft if the mercury in the bulb expands and some of the mercury is forced into the shaft. The mercury is cooler than body temperature. When the thermometer bulb is placed next to body tissue, the mercury absorbs some of the body's heat. As the mercury gets warmer, it expands. Since the mercury has no more room in the bulb, some of the mercury is forced into the shaft. More and more mercury is forced into the shaft until the mercury reaches the same temperature as the body tissue and stops expanding. The patient's temperature is determined by measuring how much the mercury expanded.


Reading the glass thermometer (that is, determining the temperature shown) is done by holding the thermometer horizontally by the stem end (the end opposite the bulb) at eye level and rotating the thermometer until the mercury in the shaft can be clearly seen. This procedure is discussed below.

a. Hold the Thermometer at eye level. You must hold the thermometer at the end of the stem, not the bulb end. (If you held the bulb end, your body heat could cause the temperature reading to increase if the temperature of your fingers is greater than the temperature shown on the thermometer.) Normally, the end of the thermometer is held with the fingertips of the right hand as shown in figure 2-1 A. The thermometer should be held at eye level to make reading easier.

NOTE: In order to assist you with reading the patient's temperature, most thermometers have an arrow pointing to the line denoting the average normal body temperature (98.6 F or 37 C).

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Figure 2-1. Reading a glass thermometer.

A Holding the thermometer.

B Rotating the thermometer.

C Reading the Fahrenheit thermometer.

D Same temperature on a Celsius thermometer.

b.  Rotate the Thermometer. The stem of the thermometer is not perfectly round. Its flattened areas act as a magnifying glass to make the hollow shaft in the stem appear larger. (The flattened areas also keep the thermometer from rolling off a flat surface.) Using your fingertips, slowly rotate the thermometer back and forth until the mercury in the shaft is clearly visible (see figure 2-1 B). The column of mercury will appear to be silver in color and the rest (empty part) of the shaft will appear whitish. The column of mercury should be visible just above the numbers.

c.  Locate the Nearest Temperature Mark. There are several temperature markings (lines) on a thermometer. Identify the marking that is closest to the point where the mercury in the shaft stops (where the silver meets the white).

d. Determine the Temperature Reading. The method you use to determine the temperature reading of the mark you selected in the previous step depends upon whether you are using a Fahrenheit thermometer or a Celsius thermometer.

(1) Fahrenheit thermometer. On a thermometer that uses the Fahrenheit temperature scale, each long mark shows a whole degree of temperature. There are four shorter lines between each pair of long lines. These shorter lines show an increase of two-tenths of a degree Fahrenheit (0.2F). For example, the first short mark past (to the right of) the 98F mark shows a temperature reading of 98.2F. The second mark shows a reading of 98.4 F. The third mark is 98.6F and the fourth is 98.8 F. The next mark is a long line and represents a reading of 99o F. In figure 2-1 C, the temperature reading is 99.4 F.

(2) Celsius thermometer. Like the Fahrenheit temperature, each long line on a thermometer using the Celsius scale denotes a whole degree of temperature. On the Celsius scale; however, there are nine shorter marks between each pair of long marks. Each small mark shows an increase of one-tenth of a degree Celsius (0.1 C). For example, the third short mark past the 38 C line denotes a temperature reading of 38.3C. Figure 2-1 D shows the same temperature as figure 2-1 C using a Celsius scale thermometer. The Celsius reading is 37.4 C.


Glass thermometers are designed so that the temperature reading will not go down (decrease) when the thermometer is removed from the patient. The temperature reading will remain unchanged unless the bulb end of the thermometer comes in contact with something hotter than the temperature reading shown on the thermometer or the thermometer is shaken down. "Shaking down" is the term given to the method of forcing mercury from the stem back into the bulb.


If a glass thermometer is not shaken down, then the thermometer continues to keep the same temperature reading. If it were to be used again on a second patient with a lower temperature, the thermometer would still show the temperature of the first patient.


a. Stand in a Clear Area. Stand in a clear area so that you will not hit the thermometer against anything during the shaking down process. Remember, the thermometer is glass and may shatter if it strikes anything hard like a table or bed frame.

b. Grasp the Stem End Securely. Grasp the end of the thermometer that is opposite the bulb end with your fingers and thumb. The stem ends in a knob (usually colored either blue or red) that helps you to hold onto the thermometer during the shaking down process.

c. "Crack the Whip." Shake the thermometer down using quick, sharp, downward wrist motions (figure 2-2). These quick wrist motions are sometimes called "cracking the whip." Perform several of these motions. You may have a problem getting effective "cracks" at first, but with practice you will soon be able to lower the temperature to the necessary level quickly.

d. Read the Thermometer. After shaking down the thermometer, read the temperature shown in order to evaluate the effectiveness of your efforts.

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Figure 2-2. Shaking down a glass thermometer.


An electric thermometer may be battery-powered or plug into an electric outlet. The operating instructions will vary from one model to another. Therefore, you must be familiar with the operating instructions for the thermometer that you will be using. Pay special attention to the "warm up" requirements.

a. Probe. The electric thermometer uses a metallic sensing device called the probe. The probe is like the mercury in a glass thermometer in that the probe absorbs heat from the body tissue that surrounds it. An electric thermometer usually has two probes. The probe that is color-coded blue is used to take oral temperatures. The probe that is color-coded red is used to take rectal temperatures. Both probes are usually the same shape and size.

b. Probe Cover. Just as the mercury in a glass thermometer never actually touches the patient, a probe is kept from touching the patient's body by a probe cover. The cover is usually made of paper and is discarded after one use. The probe cover gives the electric thermometer a great advantage over glass thermometers in that the electric thermometer is ready to be used again once that the used cover is disposed of and replaced by a new probe cover. A glass thermometer, on the other hand, must be cleaned and sterilized before being used again.

c. Display. The probe has a cord that plugs into the main body of the electric thermometer. The thermometer body displays the information obtained from the probe. An electric thermometer may have either a digital or a scale display.

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Figure 2-3. An electric thermometer.

(1) Digital display. If the electrical thermometer has a digital display, numerals showing the patient's temperature will be shown on the screen. The electric thermometer in figure 2-3 has a digital display.

(2) Numerical scale display. A numerical scale display looks much like the temperature scale of a glass thermometer. A permanent temperature scale is displayed and the patient's temperature is represented by a line. The longer the line is, the higher the temperature reading.


a.  Digital Display. On a digital display, the patient's temperature that is shown is already rounded off to the nearest two-tenths of a degree Fahrenheit or to the nearest one-tenth of a degree Celsius.

b.  Numerical Scale Display. A numerical scale display is read using the same procedures as a glass thermometer. Since you want your reading to be to the nearest two-tenths of a degree Fahrenheit or to the nearest one-tenth of a degree Celsius, you first determine which scale to use. Then find the mark on that scale that is nearest the end of the displayed line. Once the mark had been identified, determine the temperature represented by that mark on the scale.