The box was filled nearly, but not quite full, of granulated hard carbon.
In Norwood and Rogers's process a thin coating of tin is applied to the iron before it is dipped in the zinc, by putting the plates between layers of granulated tin in a wooden tank containing a dilute solution of stannous chloride, when tin is deposited on them by galvanic action.
The current from the battery used passed from the diaphragm through the granulated carbon to the metallic back of the box.
The space enclosed between the front and rear faces of the box is filled about three-quarters full of finely granulated hard carbon, which therefore lies in contact with the front and rear carbon disks of the apparatus, and also fills up the space lying between the lower edge of these disks and the curved surface of the case.
The current from the battery passes from one of the carbon disks to the other through the particles of granulated carbon which fill the space between them.