Electrical resistance is the property of a material that opposes the flow of electric current. This opposition is quantified in units called ohms (\(\Omega\)), and measuring it is a basic requirement in electrical work. Determining resistance generally falls into two categories: direct measurement and indirect measurement. Direct measurement uses a single, dedicated instrument for an immediate reading, while indirect measurement relies on measuring other electrical quantities and then performing a calculation. The choice depends on the required precision, the magnitude of the resistance, and whether the component can be isolated from its circuit.
Principles of Direct Measurement
The most common method for determining resistance involves a specialized tool, typically a digital multimeter set to the resistance function, which acts as an ohmmeter. This instrument provides a resistance value directly on its display without requiring any separate calculations. The core mechanism involves the meter generating and applying a small, known DC current to the component being measured.
The meter then measures the resulting voltage drop that occurs across the component. Using the relationship between voltage, current, and resistance, the meter’s internal circuitry instantly calculates and displays the resistance value. A digital ohmmeter uses a stable current source to inject current and a high-input impedance voltmeter to accurately sense the resulting potential difference.
A precaution for this direct method is that the component must be isolated and unpowered before the measurement is taken. If the circuit is energized, external voltage sources will interfere with the meter’s internal current, leading to inaccurate or potentially damaging results. The instrument relies on its own internal battery as the sole source of energy for the measurement process.
For measuring very low resistances, such as less than 100 ohms, a more advanced technique called the four-wire or Kelvin method is employed. This approach uses two wires to carry the test current and two separate wires to measure the voltage drop across the component. This separation eliminates the resistance of the test leads themselves from the measurement, which is a significant source of error when dealing with small resistance values.
Indirect Measurement by Calculation
The most straightforward indirect method is the application of Ohm’s Law, which states that resistance is equal to the voltage across a component divided by the current flowing through it. This technique requires an ammeter to measure the current flowing through the component and a voltmeter to simultaneously measure the voltage drop across it.
The accuracy of the Ohm’s Law method is limited because the meters themselves introduce additional resistance or draw current, slightly altering the circuit conditions. Furthermore, the component often needs to remain powered for this measurement, which can cause its temperature to rise and change its resistance value. Despite these limitations, this method provides a practical way to determine resistance, especially in operational circuits where isolation is not possible.
A higher-precision indirect technique is the use of a Wheatstone bridge circuit. This configuration is a classic null measurement device that uses a sensitive detector, such as a galvanometer, to compare an unknown resistance to three known resistances. The circuit is balanced when no current flows through the detector, indicating that the potential difference across the bridge is zero.
By adjusting a variable resistor within the bridge until the balanced condition is achieved, the unknown resistance can be precisely calculated from the ratios of the three known resistors. Because the measurement is performed at the point of zero current flow through the detector, the method minimizes errors associated with the detector’s internal resistance. Bridge circuits are useful for measuring very small changes in resistance, making them suitable for sensing applications involving strain gauges or thermistors.
Practical Selection Criteria
The decision to use a direct or indirect measurement method hinges on the specific context of the task and the required level of accuracy. Direct measurement with an ohmmeter is preferred for quick checks, general troubleshooting, and when ease of use and speed are the top priorities. This method is suitable for a wide range of intermediate to high resistance values where high precision is not necessary.
Indirect methods, particularly those involving bridge circuits, become necessary when high precision is required, such as in laboratory settings or when calibrating other instruments. The Wheatstone bridge is adept at measuring low resistance values or subtle resistance changes that would be obscured by lead resistance in a two-wire direct measurement.
Both methods have limitations that influence their selection. Direct ohmmeters can be affected by the resistance of the test leads and contacts when measuring very low resistance. Conversely, in indirect methods where current is applied, the heat generated can alter the resistance of the component, especially if it has a high temperature coefficient. Careful consideration of the resistance range and the potential for thermal effects is necessary before selecting the appropriate measurement technique.