Grenet Battery Cells occupy little space, furnish a large quantity of current, and is especially adapted for experimental purposes, or for running electro-medical devices, especially those that require a bisulphate of mercury battery. The consist of two carbon plates on either side of the central zinc plate. The battery may be kept charged for months, and activated at any time by simply depressing the nickel-plated rod which slides through the center of the cap.  When not in use, raise the zinc above the fluid, by means of the central rod. Truax & Greene - 1893.