Why Can't the Normal Ammeter-Voltmeter Method Be Used for High Resistance?

Introduction

In electrical and electronic measurements, the ammeter-voltmeter method is a straightforward principle for measuring resistance. However, when dealing with high resistance values, this method may not yield accurate results. This article explores the limitations of using the ammeter-voltmeter method for high resistance measurements and highlights the need for specialized measurement techniques.

Limitations of the Ammeter-Voltmeter Method for High Resistance

Loading Effect

The most significant issue with using the ammeter-voltmeter method for high resistance is the loading effect. The internal resistance of the voltmeter is typically much higher than that of the ammeter. When measuring high resistance, the current flowing through the circuit becomes very small. As a result, the voltmeter may not accurately reflect the voltage across the resistor due to its own internal resistance, leading to significant measurement inaccuracies.

Current Limitation

High resistance results in a very low current flowing through the circuit. The ammeter is designed to measure current effectively, but when the current is extremely low, it may not register this current accurately. This inaccuracy adds to the overall error in the calculated resistance value.

Measurement Range

Most standard ammeters and voltmeters are not designed to be used at the range required for high resistance measurements. High resistance measurements often require specialized instruments such as micro-ohmmeters or high-resistance meters. These specialized tools offer the necessary sensitivity and accuracy to provide reliable and precise readings.

Thermal Effects

During high resistance measurements, the small current flowing through the circuit may generate negligible heat. While this heat generation might not be substantial enough to affect the accuracy significantly, it can lead to thermal EMF errors, which can introduce additional inaccuracies in the measurement.

Conclusion

To ensure accurate measurements of high resistance, it is advisable to use dedicated instruments designed for this purpose. This includes digital multimeters with high-resistance ranges and megohmmeters, which can apply higher voltage to measure resistance without being affected by the loading effect.

Real-World Example

Imagine you are measuring a resistor with a typical meter and find that the internal resistance of the meter is 20 megohms, while the resistor to be measured is also 20 megohms. In this scenario, if you measure the resistance using the ammeter-voltmeter method, your meter will indicate 10 megohms, which is quite different from the real value. A perfect multimeter would not load the circuit and would provide the correct measurement.

For accurate high resistance measurements, always use equipment designed for that purpose. Using standard meters for high resistance measurements can lead to significant inaccuracies, as demonstrated by the example above. It is crucial to select the appropriate tools to achieve reliable and precise results.