Moving-coil meters

Last updated: April 2, 2009.

Need to hunt down a problem lurking in an electric circuit? You'll need a meter of some kind, maybe even an oscilloscope. Most people use digital meters these days that show readings of current, voltage, and resistance on an LCD display (they're sometimes called solid-state or electronic meters). But many of us still prefer the old kind of meter with a pointer that sweeps back and forth on a dial. Moving-coil meters, as these things are known, are still widely used in all kinds of different equipment, from airplane cockpit instruments to sound-level (VU) meters in recording studios. Let's take a closer look at how they work!

Photo: A typical voltmeter from Radio Shack. This one can measure up to 100 volts (V).

Electricity makes magnetism

Moving-coil meters work in a similar way to electric motors. If you know how one of those works, understanding a meter is easy. Either way, let's start from the beginning. If you send an electric current down a metal wire, you briefly create a magnetic field around the wire at the same time. You can't see it but it's there nevertheless—and you can make it do some very interesting things. Put a compass near a wire, switch on the current, and you'll see the needle flick off course as you do so. Switch off the current and the needle will flick again. Roughly speaking, this is the science at work in a moving coil meter: the electric current passing down a wire creates a magnetic field that makes a needle flick to one side. But how does that happen, exactly?

Inside a meter, a tight coil of copper wire, wrapped round an iron core, is mounted in between the poles of a permanent magnet. The coil has connections at either end so you can pass an electric current through it and it has a long pointer stuck to it that runs out across the meter dial. When you connect the meter into a circuit and turn on the current, the current creates a magnetic field in the coil. The field repels the magnetic field created by the permanent magnet, making the coil rotate and turning the pointer up the dial. The more current that flows through the coil, the bigger the magnetic field it creates, the greater the repulsion, the more the coil turns, and the further up the dial the pointer goes. So the pointer gives you a measurement of how much current is passing through the coil. With appropriate calibration, you can use the dial to measure the current directly.

Different types of meters

You can use moving-coil meters to measure voltage, current, or resistance—but you have to connect them up in different ways in each case.

Voltmeters

To measure voltage, you connect a meter in parallel across the two points of the circuit you want to measure. Voltage-measuring meters are called, not surprisingly, voltmeters.

Photo: This digital meter gives an instant reading of voltage, resistance, or current on an LCD display. You simply turn the dial in the center to convert it from one kind of meter to another. Currently it's set as a voltmeter and showing 1.717 volts DC (direct current). Photo by Juan Antoine King courtesy of US Navy.

Ammeters

To measure current, you place your meter in series (insert it directly into the path of the circuit). Current-measuring meters are generally called ammeters (since they measure in amps) or galvanometers (after Luigi Galvani, the Italian who famously discovered electric current by making frogs' legs twitch). If large currents are being measured, ammeters typically need an extra resistance called a shunt fitted in parallel with their terminals. Most of the current flows through the shunt, leaving only a small fraction flowing through the meter coil itself (thus protecting the mechanism). Some ammeters have dials on their box so you can measure a wide range of different currents. Turning the dial effectively switches a different-sized resistance into the measuring circuit, with bigger shunts used to measure larger currents.

Ohmmeters

You can measure the resistance of a circuit in three ways. You can use an ammeter and voltmeter to measure the current and voltage and then use Ohm's law. Or you can measure the resistance in a single operation using a slightly different design of moving-coil meter called an ohmmeter, which is effectively an ammeter with its own built-in battery. The battery provides a voltage of known size. When you place the meter probes across the resistance you want to measure, you complete a circuit and a current flows. The meter measures the size of this current, but shows it as a resistance (the dial is calibrated in ohms based on the fixed voltage of the battery inside the meter). You can make more accurate measurements of resistance using a slightly more complex type of circuit called a Wheatstone bridge.

Further reading

• Moving Coil Meters: more about the theory of meter circuits and the differences between ammeters, voltmeters, and ohmmeters from the excellent Hyperphysics site.
• Resistance measurements: A clear explanation of the different ways you can measure resistance, including the Wheatstone bridge.