Fuller Cell:—­A form of cell that is adapted to very heavy open-circuit work and also closed-circuit work where heavier currents are required than can be supplied by the gravity battery is the Fuller.  In this the electrodes are of zinc and carbon, respectively, the zinc usually being in the form of a heavy cone and placed within a porous cup.  The electrolyte of the Fuller cell is known as electropoion fluid, and consists of a mixture of sodium or potassium bichromate, sulphuric acid, and water.

The various parts of the standard Fuller cell, as once largely employed by the various Bell operating companies, are shown in Fig. 65.  In this the jar was made of flint glass, cylindrical in form, six inches in diameter and eight inches deep.  It is important that a good grade of glass be used for the jar in this cell, because, on account of the nature of the electrolyte, breakage is disastrous in the effects it may produce on adjacent property.  The carbon plate is rectangular in form, about four inches wide, eight and three-quarters inches long, and one-quarter inch thick.  The metal terminal at the top of the carbon block is of bronze, both it and the lock nuts and bolts being nickel-plated to minimize corrosion.  The upper end of the carbon block is soaked in paraffin so hot as to drive all of the moisture out of the paraffin and out of the pores of the block itself.

The zinc, as is noted from the cut, is in the form of a truncated cone.  It is about two and one-eighth inches in diameter at the base and two and one-half inches high.  Cast into the zinc is a soft copper wire about No. 12 B. & S. gauge.  This wire extends above the top of the jar so as to form a convenient terminal for the cell.

The porous cup is cylindrical in form, about three inches in diameter and seven inches deep.  The wooden cover is of kiln-dried white wood thoroughly coated with two coats of asphalt paint.  It is provided with a slot for the carbon and a hole for the copper wire extending to the zinc.

The electrolyte for this cell is made as follows: 

Sodium bichromate    6 oz. 
Sulphuric acid      17 oz. 
Soft water          56 oz.

This solution is mixed by dissolving the bichromate of sodium in the water and then adding slowly the sulphuric acid.  Potassium bichromate may be substituted for the sodium bichromate.

In setting up this cell, the amalgamated zinc is placed within the porous cup, in the bottom of which are about two teaspoonfuls of mercury, the latter serving to keep the zinc well amalgamated.  The porous cup is then placed in the glass jar and a sufficient quantity of the electrolyte is placed in the outer jar to come within about one and one-half inches of the top of the porous cup.  About two teaspoonfuls of salt are then placed in the porous cup and sufficient soft water added to bring the level of the liquid within the porous cup even with the level of the electrolyte in the jar surrounding the cup.  The carbon is then placed through the slot in the cover, and the wire from the zinc is passed through the hole in the cover provided for it, and the cover is allowed to fall in place.  The cell is now ready for immediate use.