Clemson U. Physics Tutorial: Measuring Devices

Physics Lab Tutorials

Physics is a quantitative science, relying on accurate measurements of
fundamental properties such as time, length, mass and temperature. To
ensure measurements of these properties are accurate and precise,
instruments such as meter sticks, Vernier calipers, micrometer calipers,
triple-beam balances and laboratory thermometers are often used. It is
important to understand how to properly use these devices. With any
measurement tool, the student should always try to achieve the greatest
accuracy the apparatus will allow.

Meter stick. The simplest way to measure length is to use an
ordinary meter stick. In the laboratory, our meter sticks are carefully
calibrated in centimeters with a millimeter least count. That is, the
millimeter is the smallest subdivision on the meter stick, which can be
seen in Figure 1. This means the millimeter is the unit of the smallest
reading that can be made without estimating.

Figure 1. This meter stick is calibrated in centimeters (shown as the numbered major divisions) with a least count of millimeters.	Figure 2. An example of a meter stick reading. The object’s length is measured to be 41.64 cm. (See text for description.)

Vernier caliper. A vernier caliper (or vernier), shown in Figure 3, is a
common tool used in laboratories and industries to accurately determine the
fraction part of the least count division. The vernier is convenient when
measuring the length of an object, the outer diameter (OD) of a round or
cylindrical object, the inner diameter (ID) of a pipe, and the depth of a hole.

Figure 3. The vernier caliper. A common instrument used to measure lengths, depths and diameters. (See text for description.)

The vernier consists of a main scale engraved on a fixed ruler and an
auxiliary scale engraved on a moveable jaw (see Figure 3). The
moveable jaw is free to slide along the length of the fixed ruler. The
main scale is calibrated in centimeters with the smallest division in
millimeters. The moveable auxiliary scale has 10 divisions that cover the
same distance as 9 divisions on the main scale. Therefore, the length of
the auxiliary scale is 9 mm. When the vernier is closed and properly zeroed
(see Figure 4), the first mark (zero) on the main scale is aligned with
the first mark on the auxiliary scale. The last mark on the auxiliary scale
will then coincide with the 9-mm mark on the main scale.
A reading is made by closing the jaws on the object to be measured.
Make a note of where the first mark on the auxiliary scale falls on
the main scale. In Figure 5, we see that the object’s length is between
1.2 cm and 1.3 cm because the first auxiliary mark is between these two
values on the main scale. The last digit (tenths of a millimeter) is
found by noting which line on the auxiliary scale coincides with a mark on
the main scale. In our example, the last digit is 3 because the third
auxiliary mark lines up with a mark on the main scale. Therefore, the
length of the object is 1.23 cm.

Figure 4. With the jaws closed, this is what a properly zeroed vernier caliper should look like. Click on the image to enlarge it.	Figure 5. An example of a vernier reading. The object’s length is measured to be 1.23 cm. Click on the image to enlarge it.

Figure 6. An improperly zeroed caliper. In this case, the error is positive (+0.05 cm) and is to be subtracted from the measurement reading. Click on the image to enlarge it.

Triple-beam balance. The triple-beam balance, or laboratory balance,
measures the mass of an object by balancing the unknown mass with sliding
masses of known values. The triple-beam balance is usually calibrated in
grams with a least count of 0.1g. A measurement, then, can be made to 0.01g.
It is important to note that laboratory balances are used to make measurements
of an object’s mass, not weight. (The weight of an object, as you will learn,
is the product of the object’s mass, m, and the acceleration due to gravity,
g, or W = mg.)
Before the triple-beam balance is used to make a measurement,
verify that the balance is properly zeroed. Fine adjustments may
be made by turning the knob under the balance pan.

Graduated cylinder. The volume of an irregularly shaped object
may be determined with the use of a graduated cylinder. To do so, fill the
cylinder with water or other liquid and completely submerge the object in the
liquid. The volume of the object is measured by calculating the difference
in the water level before and after the object was submerged. Graduated
cylinders are usually calibrated in milliliters or cubic centimeters
(1ml = 1cc = 1cm3) but their least counts vary from 1ml to 10ml depending
on the size of the vessel.

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Jerry Hester

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Last Modified on 01/27/2006 14:25:18.