Figure 1: original figure

Figure 2: Explanatory chart

The I measurement in the original drawing is the reading of the ammeter in Figure 2, and the I in the original drawing is the current through the power supply.

In fact, because the voltmeter is connected in parallel with the sliding rheostat, the indication of the ammeter is less than the current I through the power supply. If the measured value I of current in figure 1 is measured as the current flowing through the power supply to calculate the electromotive force e and internal resistance r of the power supply, the calculated result will inevitably be very small. Because:

E true =Uab+Ir true...①

E test =Uab+I test R test ... ②

In this connection, because the current passing through the rheostat is accurate, the rheostat can be regarded as an external circuit, and the circuit after the internal resistance of voltmeter and power supply are connected in parallel can be regarded as an internal circuit. Therefore, the measurement of R is actually the resistance after the internal resistance of voltmeter is connected in parallel with the internal resistance of power supply, which must be less than the real internal resistance of power supply.

Comparing ① and ②,

Because R is less than R, I is less than I, and the voltage Uab across the external circuit is equal, so E is less than E .. This is a simple inference. Trivial formula operation is not convenient to type here.