Physicist who built upon Volta’s work

OHM

["This German scientist's name is now a unit measuring electrical resistance, often found in circuits alongside volts and amps."]

This clue points to Georg Simon Ohm, the German physicist famous for Ohm’s Law, which describes how voltage, current, and resistance relate in electrical circuits. Since Alessandro Volta invented the first battery, Ohm’s work directly expanded on Volta’s discoveries by defining how electricity flows through conductors. The answer is simply the physicist’s last name, "OHM," which also happens to be a unit of electrical resistance named in his honor.