Simplify the following text for a Grade 9 native English-speaking person.
Original: The volt was defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power .
Simplified: At that time , the volt was defined as the potential difference ( i.e. , what is nowadays called the `` voltage ( difference ) '' ) across a conductor when a current of one ampere dissipates one watt of power .