The largest Van de Graaf generator was built by Dr. Robert J. Van de Graaf in 1931 at the Massachussetts Institute of Technology. It was originally used to research high-energy X-rays and atom smashing, but is now on permanent display at the Thomson Theater of Electricity at the Boston Museum of Science, where it is demonstrated every day. It consists of two columns each with a 4.57 m (15 ft) hollow aluminium sphere at the top. Originally the two sphere were oppositely charged which resulted in lightning-like discharges of 5 million volts. Today, both spheres are joined together and discharge to props and grounded probes, rather than discharging to each other.