It produces positive and negative ions which discharge the needle so that the needle becomes uncharged due to neutralization. Since the needle is now neutralized, its potential is the same as the original potential. Sources of error: When measuring the distance of the probe from the centre of the sphere, a piece of string with a mass attached was dropped from the sphere for locating the zero mark of the meter ruler. This may not be accurate as the sting may not be vertically below the centre of the sphere Precautions: 1. The wire which connects the needle and the electroscope cannot touch the bench or any other earthed conductor.
This is to prevent charge leakage. 2. The charged sphere must be well away from the walls and the bench top as the induced charges on these objects will upset electric field. Conclusion: Between two parallel plates: the electric potential remains constant at constant distance from the metal plates the electric potential is proportionally to the distance from the earthed plates Around a charged sphere: the electric potential remains constant at constant distance from the centre of sphere the electric potential is inversely proportionally to the distance from the surface of sphere