Charging an Electroscope by Contact

Charging an electroscope by contact is demonstrated.

Discussion

Contact between a charged object and a neutral object allows electrons to flow from one object to the other. If the excess charge on the electrophorus is positive, mobile electrons on the electroscope post and foil leaves will move to the electrophorus, partially neutralizing its excess positive charge and resulting in a positive charge on both electrophorus and electroscope; if the excess charge on the electrophorus is negative, mobile electrons on the electrophorus will move from the electrophorus to the electroscope, repelled by the excess negative charge on the electrophorus, and leaving both negatively charged. With the electrophorus used in this movie, the electrophorus is negatively charged, and so the charge on the electroscope is also negative.