[Illustration: FIG. 55.--Wheelock cell; the elements are carbon and zinc. To set up, place the grid in the bottom of the jar and fill its two troughs each about half full of mercury. Place the porous cell in position on the grid so that it sits perfectly upright, resting in the recess of the latter. The zincs stand with lower ends resting in mercury in the troughs of the grid. Into the porous cell, to a height of _only two-thirds_ full, pour solution consisting of equal parts water and sulphuric acid, by measure. Add to this 1/2 pound nitrate soda, 1 ounce chromic acid. This solution may be made up in the above proportion and kept in covered receptacle in any desired quantity, ready for use. In the outer jar for 6 × 8 size, 2-1/2 pints of water, and 1/2 gill sulphuric acid, 1 part sulphuric acid to 20 parts water, or as much sulphuric acid as it will take without boiling. When a charge becomes exhausted it may be renewed by adding sulphuric acid and salts in the proportions given above, after drawing out with syringe enough of the old solutions to make room for the additions, but the best action is obtained with entirely new solutions. Zincs must be kept thoroughly amalgamated by keeping a good supply of mercury in the troughs.] When a current is produced by a Daniell cell: 1. Copper is deposited on the copper plate; 2. Copper sulphate is consumed; 3. The sulphuric acid remains unchanged in quantity; 4. Zinc sulphate is formed; 5. Zinc is consumed. If, however, the copper sulphate solution be too weak, the water is decomposed instead of the copper sulphate, and hydrogen is deposited on the copper plate. This deposit of hydrogen lowers the voltage, hence care should be taken to maintain an adequate supply of copper sulphate. The voltage of a Daniell cell varies from about 1.07 volt to 1.14 volt, according to the density of the copper sulphate solution and the amount of zinc sulphate present in the dilute sulphuric acid. =“Dry” Cells.=--It is often necessary to use cells in places where there is considerable jarring or motion, as for automobile or marine ignition. The ordinary cell is not well adapted to this service on account of the liability of spilling the electrolyte, hence, the introduction of the so-called dry cell. A dry cell is composed of two elements, usually zinc and carbon, and a liquid electrolyte. A zinc cup closed at the bottom and open at the top forms the negative electrode; this is lined with several layers of blotting paper or other absorbing material.