An ohmmeter measures electrical resistance in ohms using Ohm’s Law. It generates an internal current flow and measures the voltage drop between two wires. An open circuit results in infinite resistance, while no resistance gives a reading of zero. Modern ohmmeters provide digital readings with greater accuracy. It should not be connected to an electrical unit with its own voltage source to avoid false readings and damage to the ohmmeter.
An ohmmeter is a device that measures the amount of electrical friction generated when electrons pass through an electrical conductor. Also known as electrical resistance, its value is expressed in units of “ohm”. This measurement is governed by “Ohm’s Law,” which states that the current flowing through an electrical circuit is directly proportional to the amount of voltage exerted. If written as an algebraic equation, this natural phenomenon would look like this: R = V/I, where R is Resistance, V is Voltage, and I represents Current. This illustration of the relationship between these values is attributed to the 19th-century German physicist and teacher, Georg Simon Ohm.
The mechanics of how an ohmmeter works are very simple. First, an ohmmeter must be capable of generating an internal current flow; therefore, it has its own battery. The device also consists of two wires, from which the resistance between them is measured. The red cable is connected to the corresponding positive terminal of the electrical unit under test, while the black is connected to the negative. As current flows from the battery and through the unit, the ohmmeter measures the voltage drop, or resistance, that occurs.
If there is an openness in the circuit, the result produced is called “infinite resistance” and is indicated by the needle of the instrument extending to the far left of the logarithmic scale. This may seem strange since most other electrical measuring devices swing to the extreme right to indicate a maximum level. On the other hand, if there is no resistance, an ohmmeter will give a reading of zero. However, if resistance was expected, this reading indicates that there is a short in the unit under test.
While the first devices used only two cables, the next generation consisted of four. One pair directed the current flow, while the other measured the resistance. This improvement was intended to compensate for any variation in voltage regulation between the first two leads that might compromise accuracy, particularly when attempting to measure very low resistance. Finally, the modern ohmmeter in use today provides digital readings with much more accuracy, a marked improvement over its analog predecessor.
Regardless of whether it is analog or digital, an ohmmeter should never be connected to an electrical unit that has its own voltage source. First, the meter is designed to measure resistance based on the current flowing from your battery. Any interference from another current source will affect its function and produce false readings. Also, if the secondary voltage source is high enough, it can irreversibly damage the ohmmeter.
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