1. Rapid polarization; 2. High internal resistance due to porous pot; 3. Restricted space for electrolyte causing rapid lowering of level of liquid by evaporation; 4. Eating away of the zinc rod at the surface of the liquid, rendering the rod useless before the lower part is consumed.
Fuller Bichromate Cell.—In the bichromate cells or the chromic acid cells, bichromate of soda, or bichromate of potassium, is used for the depolarizer, water and sulphuric acid being added for attacking the zinc.
The Fuller cell is of the two fluid type. A pyramidal block of zinc at the end of a metallic rod covered with gutta-percha is placed in the bottom of a porous cup containing an ounce of mercury. The cup is then filled with a very dilute solution of sulphuric acid or water and placed in a jar of glass or earthenware containing the bichromate solution and the carbon plate. The diffusion of the acid through the porous cup is sufficiently rapid to attack the zinc, which being well amalgamated, prevents local action; while the hydrogen passes through the porous cup and combines with the oxygen in the bichromate of potassium. This type of cell has an electromotive force of 2.14 volts, and is suited to open circuit, or semi-closed circuit work. The directions for setting up a Fuller cell are as follows:
1. To make the “electropoion” fluid, mix together one gallon of sulphuric acid and three gallons of water, and in a separate vessel, dissolve six pounds of bichromate of potash in two gallons of boiling water; then thoroughly mix together the two solutions. 2. Immerse the zinc in a solution of dilute sulphuric acid, and then in a bath of mercury, and rub it with a brush or cloth so as to reach all parts of the surface. 3. Pour into the porous cell one ounce (a tablespoonful) of mercury, and fill the porous cell with water up to within two inches of the top. 4. Place the porous cell and the carbon plate in the glass jar, as in fig. 46, and fill glass jar to within about three inches of the top with a mixture of three parts of electropoion fluid to two parts of water.
5. The zinc should be lifted out occasionally and the sulphate washed off. 6. The supply of mercury in the porous cell should be maintained, so as to have the zinc always well amalgamated. 7. To renew, clean all deposits from carbon plate and zinc, and set up with fresh solution.
The Edison Cell.—This is a single fluid cell with a solid depolarizer, as shown in fig. 48, and is well adapted for use on closed circuits.
The positive element is zinc, and the negative element black oxide of copper. The exciting fluid is a solution of caustic potash. The black oxide of copper plates are suspended from the cover of the jar by a light framework of copper, one end of which forms the positive pole of the battery. A zinc plate is suspended on each side of the copper oxide element and kept from coming in contact with the latter by means of vulcanite buttons.