How Do Ohmmeters Work?

An ohmmeter measures the electrical resistance of a circuit by sending a small charge from one terminal into the circuit, receiving the charge on its opposite terminal and calculating the loss. The ohmmeter works according to Ohm's Law, which expresses the resistance as a ratio of voltage to current. Ohmmeters are always used with the only source of voltage being the meter's battery.

To use the ohmmeter, turn it on, then turn off and unplug the appliance to be tested. Connect the test leads to the ohmmeter appropriately; usually they are color-coded. Touch the probes of the ohmmeter to the power terminals of the appliance and observe the reading. A zero reading means the appliance has a short circuit, and an infinite reading indicates an pen circuit that cannot carry electricity.

If an ohmmeter's battery is drained, the reading is compromised because the charge isn't as strong as it would be otherwise. Also, an ohmmeter should never be tested on a live circuit; it could destroy the meter because it isn't intended to take a higher current than that produced by its own power source. Although many newer ohmmeters are digital, there are still older analog versions, but these are not as precise.