Electrical resistance is a fundamental property of materials, representing the opposition they offer to the flow of electric current. This opposition stems from the collisions between moving electrons and the fixed atoms within a conductor’s structure. The standard international unit for electrical resistance is the ohm, symbolized by the Greek capital letter omega (\(\Omega\)). Resistance measurement is a foundational practice in electronics, diagnostics, and material science, where it helps ensure the proper functioning of components and systems. Technicians measure resistance to troubleshoot circuits, identify faulty components, or verify the quality of electrical connections.
The Foundational Principle of Measurement
All methods for quantifying electrical resistance are ultimately derived from the physical relationship known as Ohm’s Law. This law states that the voltage (V) across a component is directly proportional to the current (I) flowing through it, with resistance (R) acting as the constant of proportionality. Rearranging this formula shows that resistance is simply the ratio of voltage to current, expressed as R = V/I. This relationship serves as the theoretical basis for every instrument designed to measure ohms. Early attempts at resistance measurement required two separate meters, a voltmeter and an ammeter, which introduced complexity and potential compounding of measurement errors. The development of dedicated resistance-measuring tools solved this issue by integrating both measurements and the subsequent calculation.
Direct Measurement Using Ohmmeters
The most common and accessible method for measuring resistance involves using an ohmmeter, which is typically a function integrated into a digital multimeter (DMM). When a DMM is set to its resistance mode, it actively injects a small, known current into the component under test. The meter then measures the resulting voltage drop across the component, applying Ohm’s Law internally to calculate and display the resistance value.
This internal process requires that the circuit being tested must always be completely de-energized, or “out-of-circuit,” before a resistance measurement is taken. If the ohmmeter attempts to take a reading on a live circuit, the external voltage would interfere with the meter’s own injected current, leading to inaccurate readings and potential damage to the instrument.
The ohmmeter’s measurement uses the two-wire method, which is sufficient for most general-purpose measurements of medium-to-high resistances. However, this method includes the resistance of the test leads and the contact resistance at the probe tips in the final reading. For low-resistance components, such as a heating element, the lead resistance can become a significant fraction of the total measured value, compromising accuracy.
Specialized Techniques for High Accuracy
When precision is paramount, especially for resistance values below one ohm, specialized techniques are employed to overcome the limitations of the standard two-wire ohmmeter.
Wheatstone Bridge
One such technique is the Wheatstone bridge, a classic circuit configuration designed for high-accuracy measurements of medium to high resistance. The bridge consists of four resistors arranged in a diamond shape, with an unknown resistor balanced against three known resistors, one of which is adjustable. The measurement is performed by adjusting the known variable resistor until the voltage difference between the two parallel branches of the circuit is zero, a condition called the “null” point. At this null point, the ratios of the resistance values are equal, allowing the unknown resistance to be precisely calculated from the known values. The Wheatstone bridge is widely used in applications like strain gauges.
Kelvin (Four-Wire) Sensing
For measuring very low resistances, such as the contact resistance of a switch or the resistance of a busbar, the Kelvin or four-wire sensing method provides superior accuracy. This technique removes the error caused by the test leads by employing two separate pairs of connections. One pair of wires, called the “force” leads, is used to inject a constant, known current through the component. The second pair, the “sense” leads, connects directly across the component to measure the resulting voltage drop. By separating the current path from the voltage measurement path, the Kelvin method provides highly reliable readings down to the milliohm and microohm range.