Technicians, engineers and scientists use multimeters to measure electricity and electronic components. Though most multimeters made as of 2011 have digital circuitry and numeric displays, equipment makers still produce a few models with a traditional analog needle and printed scale. While getting precise measurements with an analog meter is more difficult, it is better for approximate measurements and monitoring voltages that change. You can read a needle’s movement with a glance, but a numeric display takes a moment of thought.
The heart of an analog multimeter is its electromechanical display, called a D’Arsonval movement. Its design uses a permanent magnet, a wire coil on a spring and a needle. When electric current flows through the coil, it creates a magnetic field. More current produces a stronger field. The attraction between this field and the permanent magnet twists the coil. As the coil turns, it moves a needle across a printed dial face. When you stop the current, the spring pulls the needle back to its original position.
An analog multimeter measures resistance by sending a small voltage across its probes. The current flowing back into the meter moves the needle. You select one of the analog meter’s three or four resistance ranges by turning its function knob. A meter has ranges of ohms x 1, ohms x 100 and ohms x 1,000. Before you measure resistance, you “zero” the meter by touching its probes together and adjusting a thumb wheel control until the needle points to zero. When you touch the probes to the resistor, the needle points to a number. To find the resistance, you multiply this number by the range multiplier: 1, 100 or 1,000.
An analog multimeter measures voltage in four or five ranges. As with resistance, you select the appropriate range by turning the function knob. The meter measures alternating current (AC) and direct current (DC) voltages in separate ranges, so if you measure voltage, you need to know if the source is AC or DC. You measure batteries and other low-voltage parts on the lowest scale, up to 10 volts. Higher ranges measure higher voltages. Multimeters usually have a maximum voltage rating of 500 volts. Higher voltages may damage the meter.
A typical analog multimeter measures current in a few different ranges, from 10 milliamps to 10 amps. The highest range uses a different internal circuit, so you plug the probe leads into different sockets on the meter to use this circuit. The lower ranges are fuse-protected, so if you measure a large current by mistake, it blows an inexpensive fuse. Analog meters cannot measure negative currents directly. The needle cannot move more than a few millimeters left of the zero mark if a current flows in the wrong direction. The meter indicates the negative current flow as positive and gives you an accurate reading if you switch the positive and negative probes.
Chicago native John Papiewski has a physics degree and has been writing since 1991. He has contributed to "Foresight Update," a nanotechnology newsletter from the Foresight Institute. He also contributed to the book, "Nanotechnology: Molecular Speculations on Global Abundance."