Calculate the internal resistance of the power-pack from the following results:
| Resistance in External Circuit (ohms) | p.d. across the source (Volts) |
| 0 | 3.90 |
| 22 | 2.40 |
| 180 | 2.94 |
| 330 | 3.01 |
| 560 | 3.08 |
| 1000 | 3.11 |
