Daniell cells are used especially for electroplating, electrotyping and telegraphic work. The electromotive force of a single cell is 1.079 volts. =Directions for Making a Daniell Cell.=--The simple Daniell cell shown in fig. 52 may be easily made as follows: The outer vessel A, consists of a glass jar (an ordinary glass jam jar will do) containing a solution of sulphuric acid (1 part in 12 to 20 parts of water), and a zinc rod B. Inside the jar is placed a porous pot C containing a strip of thin sheet copper D, and a saturated solution of sulphate of copper (also called “blue stone” and “blue vitrol”). The zinc is preferably of the Leclanche form, which will be found to be cleaner, more durable, and cheaper than a zinc sheet. The porous pot should be dipped in melted paraffin wax, both top and bottom, to prevent the solution mingling too freely and “creeping.” A few crystals of copper sulphate are placed in the pot as shown. [Illustration: FIG. 52.--Simple Daniell cell for closed circuit work. To maintain a constant current for an indefinite time, it is only necessary to maintain the supply of copper crystals and zinc. The cell as shown in the figure is easily made by following the direction given in the accompanying text.] In mixing the sulphuric acid and water, _the acid should be added to the water--never the reverse._ Zinc sulphate is sometimes used instead, as it reduces the wasteful consumption of the zinc, but it should be pure. With care the cell will last for weeks. When it weakens or “runs down,” an addition of sulphuric acid to the outer jar and a few more crystals placed in the porous pot will put the cell in good condition. [Illustration: FIG. 53.--Daniell gravity cell, “crowfoot” pattern. This is a two fluid cell in which gravity instead of a porous cup is depended upon to keep the liquids separate. The two solutions consist of copper sulphate and dilute sulphuric acid, the elements being made of zinc and copper.] =Gravity Cells.=--In a two liquid cell, instead of employing a porous cell to keep the two liquids separate, it is possible, where one of the liquids is heavier than the other, to arrange that the heavier liquid shall form a stratum at the bottom of the cell, the lighter floating upon it. Such arrangements are called _gravity cells_; but the separation is never perfect, the heavy liquid slowly diffusing upwards. =Daniell Gravity Cell.=--In this cell, shown in fig. 53, the same elements are used as in the ordinary Daniell cell, but the porous pot is dispensed with, the two solutions being separated by the action of gravity as explained in the preceding paragraph.