Investigating the Internal Resistance of a Cell

The internal resistance of most sources of electrical power, such as D.C. "power packs" and batteries, is fixed and cannot be changed. The design of these cells allows the internal resistance to be easily changed.

This experiment can be done with either a fuel cell or the aluminium/air cell. If you have both the aluminium/air cell is obviously the safer and simpler choice.

The internal resistance can be changed in two ways. The electrolyte concentration can be changed, as can the separation of the electrodes. Altering the separation is probably the easiest to do, as all that is needed is some pieces of card or plastic, as shown in the diagram.

The circuit for measuring the internal resistance is given below.

The current is set to some convenient value, such as 10 mA, or 50 mA, and the "voltage drop" is calculated. (The difference between open circuit voltage and the voltage at the specified load current.) This is used to find the internal resistance, IR = voltage drop/ current.