Complex sentence: In 1929, Van de Graaff developed his first generator with help from Nicholas Burke (producing 80,000 volts) at Princeton University; by 1931, he had constructed a much larger generator, capable of generating 7 million volts.
Split into simpler sentences: In 1929, Van de Graaff developed his first generator (producing 80,000 volts) with help from Nicholas Burke at Princeton University. By 1931, he had constructed a larger generator, generating 7 million volts.