The Vernier caliper consists of a main scale on a fixed ruler and another scale, called the auxiliary, on a movable jaw. The movable auxiliary scale slides along the fixed ruler. Distances are marked in centimeters and millimeters. Read the distance by examining the mark on the auxiliary scale and then obtain the final figure from the main scale, all as shown on Figure 4 on the Clemson University reference site.
The Vernier Caliper for Mass, Volume & Density
Vernier calipers are instruments used primarily in laboratory settings to measure specific distances, such as the outer diameter of a round object. Mass, volume and density are interrelated, as density is equal to mass divided by volume. In order to obtain the numbers for this equation, the Vernier caliper becomes a necessary tool in the laboratory.

Operation
Mass

Mass represents the amount of matter that a substance contains, A substance's mass is a constant and is not to be confused with its weight. Weight is based upon gravitational pull, with weightlessness being experienced in space by astronauts who still maintain their same mass. For objects in the laboratory on Earth a scale measures mass which will coincide with weight.

Volume

Calculating volume involves the use of the Vernier caliper. The radius and the height of a given cylinder are needed for this calculation and the Vernier is the tool for determining those calculations. Once the measurement are known then the formula for calculating volume can be completed.
Density

Obtain the density of an object by dividing the mass by the volume. Most every substance has a different density.

References
 Photo Credit measurement of diameter image by Witold Krasowski from Fotolia.com