Thevenin's theorem is the largest idea in this chapter, and it is startlingly generous: any network of sources and resistances, however tangled, behaves at one pair of terminals exactly like a single voltage source in series with a single resistance. Not approximately — exactly, for every load you could connect. The source is $V_{Th}$, the Thevenin voltage, and the resistance is $R_{Th}$, the Thevenin resistance. Whatever is inside the box stops mattering.

Better still, you can measure both with a meter and never open the box. Read the terminals with nothing connected and you have $V_{OC}$, the open-circuit voltage, in volts — and with no current flowing, nothing is dropped internally, so $V_{OC}$ is $V_{Th}$ exactly. Then clip on a known test load $R_L$, in ohms, and read the terminals again: $V_L$, the loaded terminal voltage, in volts. It will be lower, because now a current flows and the internal resistance is taking its cut.

That sag is the measurement. $R_{Th} = R_L\left(\dfrac{V_{OC}}{V_L} - 1\right)$ — read aloud R-Th equals R-L, times V-O-C over V-L, minus one. Follow it in words if the algebra feels abstract: the load current is $V_L / R_L$; the volts that went missing are $V_{OC} - V_L$; and those missing volts, divided by that current, are the ohms they were lost in. Same answer, and it is Ohm's law both times.

The $-1$ is where marks go. Without it you have computed the WHOLE loop resistance, source plus test load, and quoted it as the source's alone. The sanity rail: a battery that sags a little under load has a small $R_{Th}$, and one that collapses has a large one. That is exactly the test a shop does on a starting battery, and it is why the answer is called internal resistance in the trade.