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Concept
Ohmmeter
Summary
An ohmmeter is an electrical instrument that measures electrical resistance (the opposition offered by a circuit or component to the flow of electric current). Multimeters also function as ohmmeters when in resistance-measuring mode. An ohmmeter applies current to the circuit or component whose resistance is to be measured. It then measures the resulting voltage and calculates the resistance using Ohm’s law .
An ohmmeter should not be connected to a circuit or component that is carrying a current or is connected to a power source. Power should be disconnected before connecting the ohmmeter. Ohmmeters can be either connected in series or parallel based on requirements (whether resistance being measured is part of circuit or is a shunt resistance.)
Micro-ohmmeters (microhmmeter or micro ohmmeter) make measurements of low resistance. Megohmmeters (also a trademarked device Megger) measure large values of resistance. The unit of measurement for resistance is the ohm (Ω).
The first ohmmeters were based on a type of meter movement known as a 'ratiometer'. These were similar to the galvanometer type movement encountered in later instruments, but instead of hairsprings to supply a restoring force they used conducting 'ligaments'. These provided no net rotational force to the movement. Also, the movement was wound with two coils. One was connected via a series resistor to the battery supply. The second was connected to the same battery supply via a second resistor and the resistor under test. The indication on the meter was proportional to the ratio of the currents through the two coils. This ratio was determined by the magnitude of the resistor under test. The advantages of this arrangement were twofold. First, the indication of the resistance was completely independent of the battery voltage (as long as it actually produced some voltage) and no zero adjustment was required. Second, although the resistance scale was non linear, the scale remained correct over the full deflection range. By interchanging the two coils a second range was provided.
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