Measuring low resistance accurately requires specific techniques to minimize the impact of lead and contact resistances. Several methods are available, each suited to different resistance ranges and accuracy requirements. Here's a breakdown of common techniques:
Ammeter-Voltmeter Method
This is the simplest method, but it's crucial to connect it properly to minimize errors, especially when measuring low resistances.
-
Principle: Ohm's Law (R = V/I)
-
Procedure: Measure the current (I) flowing through the resistance and the voltage (V) across it using an ammeter and voltmeter, respectively. Calculate the resistance (R) using the formula R = V/I.
-
Considerations:
- Lead Resistance: The resistance of the connecting leads can significantly affect the accuracy when measuring low resistances.
- Voltmeter Loading Effect: The voltmeter draws some current, which can affect the current measurement.
-
Connection for Low Resistance Measurement: Connect the voltmeter directly across the low resistance to minimize lead resistance affecting the voltage measurement. This method is most suitable when the resistance of the voltmeter is significantly higher than the resistance being measured.
Kelvin's Double Bridge Method
Kelvin's Double Bridge is a more precise method specifically designed for measuring low resistances.
-
Principle: A modification of the Wheatstone bridge that eliminates the effect of lead and contact resistances.
-
Working: It uses a second set of ratio arms to connect the standard resistor to the unknown resistor, effectively eliminating the effect of lead resistance.
-
Advantages:
- High Accuracy: Significantly reduces errors caused by lead and contact resistances.
- Suitable for very low resistances (down to micro-ohms).
-
Disadvantages:
- More complex setup compared to the ammeter-voltmeter method.
Four-Point Probe Method (Kelvin Sensing)
This method is widely used for measuring the sheet resistance of thin films and semiconductor materials. It's also suitable for low resistance measurements in general.
-
Principle: Separates the current-carrying leads from the voltage-sensing leads.
-
Procedure: Four probes are placed in contact with the material. A current is forced through the outer two probes, and the voltage is measured between the inner two probes.
-
Advantages:
- Eliminates the effect of lead and contact resistances.
- Relatively simple to implement.
-
Applications:
- Semiconductor characterization
- Thin film measurements
Micro-ohmmeter
A micro-ohmmeter is a specialized instrument designed to measure very low resistances with high accuracy.
-
Principle: Typically uses a four-terminal measurement technique (Kelvin sensing).
-
Features:
- High resolution (e.g., micro-ohm or even nano-ohm resolution).
- Current source to force a known current through the resistance.
- Voltage measurement circuitry to accurately measure the voltage drop.
-
Advantages:
- High Accuracy and Precision
- Easy to use for quick measurements.
-
Disadvantages:
- More expensive than other methods.
Resistance Bridge (Wheatstone Bridge)
Although less suited for very low resistances than the Kelvin bridge, it can be used with careful attention to lead resistance.
- Principle: Compares the unknown resistance with a known standard resistance.
- Procedure: The bridge is balanced by adjusting known resistances until the galvanometer shows zero deflection. The unknown resistance can then be calculated.
- Limitations for Low Resistance: Lead and contact resistances can introduce significant errors. It's generally not the best choice for resistances significantly below 1 ohm unless special precautions are taken.
| Method | Resistance Range | Accuracy | Complexity | Cost |
|---|---|---|---|---|
| Ammeter-Voltmeter | Relatively High | Low | Low | Low |
| Kelvin's Double Bridge | Low to Very Low | High | Medium | Medium |
| Four-Point Probe | Low to Medium | Medium to High | Medium | Medium |
| Micro-ohmmeter | Very Low | High | Low | High |
| Wheatstone Bridge | Medium to High | Medium | Medium | Low |
In summary, the choice of method depends on the desired accuracy, the available equipment, and the resistance range being measured. For the most accurate measurements of low resistances, Kelvin's Double Bridge, Four-Point Probe, or a Micro-ohmmeter are generally preferred.
