## Physics: Principles with Applications (7th Edition)

See Figure 19–2. Connect the battery to a known resistance R and measure the terminal voltage $V_{ab}$. By Ohm’s law, the current is $I=\frac{V_{ab}}{R}$. This may be combined with equation 19-1 to solve for the internal resistance, $r = R\frac{\mathcal{E}- V_{ab}}{V_{ab}}$.