In Tesla Coil, attached to the Exploratorium donation barrels, causes a spark to cross an air gap with a snapping sound. Visitors trigger the Tesla Coil by blocking a light beam or pressing a switch. All the elements of the coil are in plain view and high above the visitor for safety reasons and in order to provide a clear view. When the arc occurs, emissions from the arc cause a nearby fluorescent tube to glow. Tesla Coil is a popular exhibit, common to many science centers. Tesla coils are resonant transformers: energy, in a constant balance. moves from inductance to capacitance and back again.