In the crowfoot type of gravity cell, a star-shaped group of copper sheets, surrounded. by crystals of copper sulphate (CuSO4), is placed in the bottom of a glass jar. Water to which a few drops of sulphuric acid have been added is poured in until the cell is nearly full, and then a zinc plate or "crowfoot" is hung from the upper edge of the jar.
As the sulphuric acid acts upon the zinc, zinc sulphate is formed; this is less dense than copper sulphate, and they keep separate in the gravity cell without the porous cup.
Since polarization does not take place in the gravity cell, it maintains a practically constant difference of potential at its terminals, and is capable of giving a nearly constant current. This is the cell ordinarily used for telegraphic work.
When this cell is in good condition, the blue solution of copper sulphate should fill the jar to a little above the middle, and the line of separation between it and the zinc sulphate solution should be clearly defined. If the cell is unused for some time, the copper sulphate solution will reach the zinc, and copper will be deposited upon it. The cell can be brought back to its proper condition by short-circuiting it, that is, connecting the two terminals by a short piece of wire, for a few hours As the water evaporates, the zinc sulphate crystallizes in the form of white crystals around the edge of the jar, and unless this is coated with paraffin, the crystals will form over the top and down the outside of the jar.
