Two brass arms in the shape of composite arches, each with a brass ball at one end, may be held by means of insulating glass and wood handles, and rotated on a pin so as to reduce or widen the gap between them.

If one of the contacts of a charged capacitor is placed against one end of the arch and the other end is brought close to the second terminal, a powerful spark passes between the latter and the arch. The explanation is that when the end of the arch touches the capacitor, which we we may suppose is positively charged, the whole arch is charged with the same sign. When the other end of the arch is brought near to the negative terminal of the capacitor, an electrical discharge passes through the thin layer of air due to the sudden recombination of opposite charges.

If this procedure is repeated several times, the sparks become progressively less intense and eventually stop, indicating that the capacitor has been completely discharged.