The simple physical background representative a simple circuit in which a battery of emf ε and internal resistance Ri drives a current I through an external resistor of resistance. The external resistor is usually referred to as the load resistor R. It could stand for either an electric light, an electric heating element, or, maybe, an electric motor. The basic purpose of the circuit is to transfer energy from the battery to the load. If we connect the source to the load R, the current I is
Figure 1 Equivalent circuit of the DC source
Figure 2 Electric circuit of DC source with the load R
The internal resistance Ri causes the terminal voltage USV = V1– V2 between points 1 and 2 (Fig. 2) to depend on the current
We can use eq. (2) to determine the electromotive voltage ε and internal resistance Ri measuring the loading characteristics USV = f(I). After rearrangement eq. (1), we obtain another useful expression
Eq. (3) makes it possible to determine the electromotive voltage ε and internal resistance Ri measuring the loading characteristics 1/I = f(R).
Figure 3 Dependence of the terminal voltage USV on the current I, with the linear fit
Figure 4 Dependence of the inverted value of the current 1/I on loading resistance R
 HALLIDAY, D., RESNICK, R., KRANE, K.S.: Physics : extended version. Volume two – 4th ed. New York : John Wiley & Sons, Inc., 1992. 1216 p. ISBN 0-471-54804-9.
 SCHAUER, F., KREMPASKÝ, J.: Fyzika I a II, Zlín : Univerzita T. Bati, 2004. 611 s.