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Ohmmeters measure resistance using Ohm’s law. The red and black leads are connected to the conductor’s terminals, and the resistance is indicated by a needle on the scale. Ohmmeters should not be connected to a voltage source. Analog meters must be calibrated, while digital ones can self-calibrate. Ohmmeters can also check continuity and detect open or short circuits.
Ohmmeters are electrical devices used to measure the resistance of a particular conductor. This measuring instrument works on the basis of Ohm’s law, which applies to electrical circuits. According to this law, the current (I) flowing between two points in a conductor is directly proportional to the voltage (V) or potential difference between the two points. It is also inversely proportional to the resistance (R) between them. So, mathematically, V = IR.
To measure the resistance of a particular conductor, the red and black leads of an ohmmeter are connected to the conductor’s positive and negative terminals, respectively. The resistance of the wire or circuit is indicated by a needle sliding across the device’s scale. These meters measure resistance in Ohms, denoted by the capital Greek letter omega or Ω.
An ohmmeter should never be connected to a voltage source, as it can damage the equipment. This is because the device already has a source that supplies voltage to measure the resistance of a particular conductor. Resistance is measured based on the voltage drop across the conductor. In an analog meter, the far left of the scale indicates infinite resistance and the far right indicates zero resistance.
A simple analog device consists of a battery, which is the voltage source, connected to a mobile meter. A variable resistor is also connected in series with this combination so that the needle shows exactly full scale deflection and does not slip past the zero resistance mark. This resistor also limits the current and corrects for the voltage drop caused by battery aging. Analog ohmmeters must be calibrated before use, while digital ones can usually self-calibrate.
To calibrate an analog meter, both leads must be held together. The adjustment knob helps in setting the variable resistor. It must be rotated by hand so that the needle indicates zero resistance; in other words, the needle is now on the far right. This step is known as “zeroing” the meter and should be repeated each time before measuring the resistance of any wire or circuit. In the case of a digital device, holding the leads together will indicate 0, which is sufficient for its calibration.
In addition to measuring resistance, ohmmeters can be used to check the continuity of an electrical connection. For example, if the needle rests on infinite resistance at the far left of the scale, it does not indicate continuity in the circuit. This implies that there is an open point in the circuit. On the other hand, if the value of the measured resistance is zero or much lower than the expected value, it implies a short in the circuit.
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