Reasons for errors caused by internal and external wiring methods of ammeter
Ammeter manufacturers specialize in producing and manufacturing ammeters, voltmeters, hertz meters, and other instruments. Even for voltmeters and ammeters that are highly accurate in measurement, there is an error between the measured value and the true value of resistance due to the internal resistance of the voltmeter and ammeter. The volt-ampere method for measuring resistance is based on Ohm’s law for partial circuits.
When using the internal connection method, the ammeter is connected in series with the resistor, and the reading on the ammeter is the current value of the resistor. However, the voltmeter measures the total voltage of the resistor and the ammeter, so the measured value is greater than the true value. In fact, the measured resistance value is the total resistance value of the resistor and the internal resistance of the ammeter in series. If the value of the resistor Rx is much larger than the internal resistance RA of the ammeter, the voltage taken by the ammeter is very small. At this time, the voltage measured by the voltmeter is close to the voltage across the resistor, so the internal connection method is suitable for measuring large resistors.
When using the external connection method, the voltmeter is connected in parallel with the resistor, and the reading on the voltmeter is the voltage across the resistor. However, the ammeter measures the total current flowing through the resistor and the voltmeter, so the measured value is smaller than the actual value. In fact, the measured resistance value is the combined resistance of the internal resistance of the voltmeter and the resistor in parallel. If the value of the resistor Rx is much smaller than the internal resistance RV of the voltmeter, the current drawn by the voltmeter is very small. At this time, the current measured by the ammeter is close to the current flowing through the resistor, so the external connection method is suitable for measuring small resistors.