How to select the internal and external method of ammeter

I believe that everyone will use the ammeter when the question will be how to choose the internal method? The following is a summary of several methods for your reference:

A: error method ammeter internal connection R = Rx + RA, the cause of the error is the ammeter voltage, its measurement results are large, the relative error is RA/Rx; current meter external method R measurement = RxRV / (Rx + RV), error The reason is that the voltmeter is shunted and its measurement result is too small. The relative error is Rx/RV.

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

Two: The heuristic method The ammeter is tested once according to the in-line method and the external method. The counts of the voltmeter and the ammeter are changed. The relative sizes of the two tables are compared: ΔI/I and ΔU/U, where I and U are Take a set of measurements. Analyze one of the bigger reasons and avoid it. For example, ΔI/I>ΔU/U indicates that the two connections have a greater influence on the current. The reason is that the voltmeter has a well-defined partial pressure and should be avoided in the experiment. Therefore, the ammeter connection method should be used.

This method is suitable for not knowing the resistance value to be measured and the internal resistance of the voltmeter and the ammeter.

Three: the precise method of ammeter internal R method = Rx + RA, ammeter external method R = RxRV / (Rx + RV), if in the experiment know the voltmeter or ammeter accurate internal resistance value, the internal resistance of the meter can be Substituting the measurement results into a more accurate resistance of the resistance to be measured. At this time, it is not limited by the size of the error in the method, as long as you choose the connection method that can measure the exact value of the resistor.

This method is suitable for knowing the resistance value to be measured and the accurate value of the voltmeter and the current meter internal resistance.