Internal Resistance
Electricity and Magnetism

Internal resistance of a potato cell


Class practical

An introduction to the concept of internal resistance, using a more interesting example than a battery.

Apparatus and Materials

For each student group

  • Digital multimeters, 2
  • Leads, 4 mm, 5
  • Cells, 1.5 V type C, 4
  • Resistors of a range of values from 10 ohms to 100 ohms
  • Crocodile clips, 10 pairs

Health & Safety and Technical Notes

Biology teachers should note the potato cell in this experiment refers to a whole potato not an individual potato cell.

Read our standard health & safety guidance


  1. Make your potato cell. Insert the copper and zinc electrodes at either end of the potato. Attach a 4 mm lead to each electrode using a crocodile clip.
  2. Set up the circuit as shown. Set the resistance substitution box to 4.7 kΩ.. This is the load resistance. Record the current and potential difference values in a suitable table.
  3. Change the load resistance and record the values of current and potential difference. Repeat this process to gather data for a range of load resistances. You will have to change the range of your ammeter. Take care not to confuse amps with milliamps or microamps!
  4. Plot a graph of V against I. Describe the trend.

Teaching Notes

  • This experiment can be used for a number of purposes – as an introduction to the concept of internal resistance, an interesting example of internal resistance or an example of a simple cell. If standard resistors are available it is possible to vary the load resistance in smaller steps.
  • The VI graph line will surprise students who have not been introduced to the concept of internal resistance. Those students familiar with the equation V=εIr should be able to interpret the data in terms of the internal resistance of the potato cell. However, many students find internal resistance a difficult concept and may find the Internal resistance of a shoe box cell experiment a useful support activity...
  • Internal resistance of a shoe box cell

  • If readings are entered into a spreadsheet it is easy for interested students to plot further graphs, including load resistance/power dissipated in resistor. Such a graph will show a peak power output when the load resistance is equal to the internal resistance of the cell.

This experiment comes from AS/A2 Advancing Physics. It has been re-written for this website by Lawrence Herklots, King Edward VI School, Southampton. This experiment was safety-tested in June 2007

Internal Resistance
appears in the relation ε=V+Ir
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