This instrument is a flat condenser. There are two conducting discs, called "armatures", on one of which there is an insulating layer of sealing wax This makes it possible to "condense" or accumulate electrical charges. After charging the lower disc by contact, the other, initially uncharged, disc is put on top of it. This second disc gets an opposite charge by induction. When earthed, the apparatus becomes "capable" of accumulating a large amount of charge. Instruments of this type became popular in the second half of the 18^th century and were the subject of experiments and interpretation by many "Electrical Physicists", such as Franklin, Aepinus and others. Volta announced it was an original piece of apparatus, arising out of his electrophorus. He gave it its present name and claimed he invented it. He explained it functioned on the basis of his theory of electrical atmospheres.