are limited by available technology, and ultimately by inherent thermal noise. Power supply Analog meters can measure voltage and current by using power from the test circuit, but require a supplementary internal voltage source for resistance testing, while electronic meters always require an internal power supply to run their internal circuitry. Hand-held meters use batteries, while bench meters usually use mains power; either arrangement allows the meter to test devices. Testing often requires that the component under test be isolated from the circuit in which they are mounted, as otherwise stray or leakage current paths may distort measurements. In some cases, the voltage from the multimeter may turn active devices on, distorting a measurement, or in extreme cases even damage an element in the circuit being investigated. See also Electronic test equipment Electricity meter References External links Electronic test equipment Electrical test equipment Electrical meters Electronics work tools Measuring instruments