An ohmmeter is an instrument that is used to measure electrical resistance. Ohmmeters are used principally for checking the values of resistors and for detecting breaks or failures of components in electrical circuits.

An ohmmeter consists of an incomplete electrical circuit. The circuit is completed when the component being tested is connected between the leads attached to the instrument. The component to be tested must be isolated or must be part of a circuit that is not operating. Otherwise, the meter may be damaged or an inaccurate result may be obtained.

Inside an ohmmeter the circuit consists essentially of a battery, a resistor, and an ammeter, which measures the current that flows when the circuit is completed. This current is inversely proportional to the total resistance in the circuit. The resistance of the circuit ,inside the meter is known, and therefore the resistance of the object being tested can be calculated. The meter scale is marked in ohms to indicate the resistance of the object being tested. The zero point on the scale is obtained by connecting the leads directly together so that the current flows through the meter circuit only. The remainder of the values can be found by calibrating the instrument, using a set of standard resistors.

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