A: Outer shell or casing made preferably of sheet brass, elliptical in form. B: Wooden block containing cavities or recesses to receive the glass cells C, C'. C, C': Glass cells consist of two cylindrical vessels connected by a transverse tube D. E: Piece of muslin or other suitable cloth. F: Cork pierced with apertures G. H: Connecting wires. I: Copper wires, connected to H. J: Suitable binding posts, provided with covering caps L. K: Cover of the shell A, which is made of rubber.