Many years ago the GPO had "Coils Testing" for individual dry cells (usually large ones such as Flag cells (R40 and the like)). It had a resistive load connected via a press button. You connected it across the cell and a voltmeter. First of all you measured the emf directly on the voltmeter, then pressed the button and measured the pd to enable a sort of "measurement" of internal resistance. You held the load button down for, ISTR, a minute and again read the meter (after releasing the button) - this time to see the effect of polarisation (gas building up on the internal electrodes). By reference to tables and, more effectively, experience you got a good idea of the condition of the cell.
There - I've just looked it up - the load applied by the testing coil is 2 ohms.
As described:-
(i) Measure the open circuit emf of the cell: let this be E1 volts.
(ii) Connect a 2 ohm resistor across the cell and measure the terminal pd - V volts.
(iii) Remove the resistor from across the cell after one minute and measure the emf immediately the resistor is disconnected:- call this E2 volts.
The value of V and E1 enable the internal resistance r to be calculated, for the current is I = E1/(2+r) and the voltage drop across the internal resistance is Ir = E1r/(2+r) = (E1 - V)
from which r may be found.
The difference between E1 and E2 indicates the back-emf of polarisation. E2 should not be less than 1·2V.