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D'Arsonval Movement
All analog multimeters display measurements with some version of the D'Arsonval movement, based on a design that dates back to the late 1800s. The mechanism consists of a spring-loaded needle pointer resting on a wire coil. A circular magnet surrounds the coil. As a current flows through the coil, it produces magnetic force that pushes against the magnet and twists the coil, rotating the needle a few degrees. The more current in the coil, the greater the twist. When the current stops, the force disappears, and the spring returns the needle to its resting position.
Dial Face
The indicator needle moves across a printed dial face that has precision markings for voltage, current and resistance scales. To read the display, you frequently need to do a little interpolation, as the needle seldom rests on an exact mark. For example, you measure the voltage of a battery. The needle moves past the main mark for one volt and also past three minor marks, but not quite to the fourth minor mark. If you are sharp-eyed, you might interpret this as 1.38 volts. Reading the first two digits is easy; the third takes practice.
Manual Zero Adjust
A digital multimeter always automatically determines the zero point for any scale. You must manually adjust the resistance scale for zero on an analog meter, however. The analog multimeter has multiple resistance scale settings, such as ohms x 1, ohms x 100 and ohms x 1000. If you change the scale, and as the internal battery ages, you must re-zero the meter by touching the probe tips together and turning a small thumbwheel until the needle points to exactly zero ohms. Once adjusted for the scale setting, you can measure resistance with confidence.
Parallax Mirror
The needle hovers a fraction of an inch over the dial face. This spacing can lead to a source of error called parallax. If you read the meter by facing it directly, you do not have this problem, but if you read it at an angle, the needle appears to point to the left or right of its true reading. A mirrored arc behind the dial face shows a reflection of the needle; if it appears you see two needles, you find the dial reading between them to eliminate the parallax error.
Damped Response
Because of the mass of the needle and its mechanism, it has a certain motion damping due to its mechanical inertia. This damping gives analog meters an advantage over digital models: The needle moves only slightly with a noisy voltage, whereas a digital display shows an erratic, confusing jumble of changing numbers. Likewise, for slowly-changing voltages and currents, the analog needle moves smoothly back and forth across the dial, making it easy to interpret. Most digital displays race through a series of numbers which, again, is hard to read.