Analog resistance meters are instruments that measure electrical resistance. They display values on a dial, usually with a needle or moving pointer. Resistance, which is measured in ohms (W), is the opposition to the flow of electric current of a conductor. Typically, analog resistance meters are powered by an internal battery that applies a small voltage to the circuit being tested. The positive lead is connected to the circuit’s positive side and the negative lead is connected to the circuit’s ground. The current is then measured and the resistance calculated. Zero resistance indicates a short. Infinite resistance indicates an open. Often, resistance readings that exceed product specifications are caused by a faulty component or problems such as burnt contacts, corroded terminals, or loose connections. To maintain accuracy, analog resistance meters require regular calibration that consists of connecting the two leads together and zeroing the meter with the adjustment knob.
Analog resistance meters are available with a variety of features. For example, battery powered devices can be operated without plug-in power. Temperature compensated devices, which provide programming or electrical components, are designed to counteract known errors caused by temperature changes. Analog resistance meters with a mirrored scale improve readability by enabling users to avoid parallax errors. Devices with a range switch allow users to select the range of units to measure. Analog resistance meters with overload protection and diode testing are also available. Some devices are handheld and portable. Others are designed for benchtop or shop floor use.
Many analog resistance meters provide protection against stray resistances, common-mode signals, and electromagnetic interference (EMI). Guard rings, which improve the resistivity measurements of many materials, provide an additional conductive guard between a device’s low terminal and ground. When properly connected to a guard terminal, guard rings shunt common mode currents away from the measuring circuit and eliminate the surface resistance path. Guard rings are often used in analog panel meters that perform high resistance measurements in units such as megaohms. Other guarding mechanisms for analog panel meters include printed circuit board (PCB) traces and ground planes. Placing a metal enclosure around both the analog resistance meter and the test circuit also reduces EMI.
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