Selection of internal and external connection methods of ammeter

　　Method 1: Error method

　　The ammeter is connected internally by R measurement = Rx + RA. The error factor is the voltage division of the ammeter, so the measurement result is too large, and the relative error is RA/Rx. The ammeter is connected externally by R measurement = RxRV/(Rx+RV). The error factor is the current division of the voltmeter, so the measurement result is too small, and the relative error is Rx/RV.

　　This method is suitable for roughly knowing the resistance value to be measured and the internal resistance value of the voltmeter and ammeter.

　　Method 2: Trial and error

　　Measure the ammeter once by internal connection and external connection respectively. The counts of the voltmeter and the ammeter will change. Compare the relative size of the change of the two meters △I/I and △U/U, where I and U are randomly selected. Analyze a larger factor and then avoid it. For example, △I/I>△U/U, which means that the two connection methods have a greater impact on the current. The factor is that the voltage division effect of the voltmeter is obvious, which should be avoided in the experiment. Therefore, the internal connection method of the ammeter should be used at this time.

　　This method is applicable when the resistance value to be measured and the internal resistance value of the voltmeter and ammeter are unknown.

　　Method 3: Precision method

　　The ammeter is connected internally: R = Rx + RA, and the ammeter is connected externally: R = RxRV / (Rx + RV). If the accurate internal resistance of the voltmeter or ammeter is known in the experiment, the internal resistance of the ammeter can be substituted into the measurement result to calculate a more accurate resistance value of the resistor to be measured. At this time, you are not restricted by the error size in method 1, and you only need to choose a connection method that can measure the accurate value of the resistor.

　　This method is applicable to knowing the resistance value to be measured and the accurate value of the internal resistance of the voltmeter and ammeter.