[Illustration: FIG. 54.--Partz acid gravity cell. In this form of cell, the electrolyte which surrounds the zinc is either magnesium sulphate or common salt. The depolarizer is a bichromate solution which surrounds the perforated carbon plate located in the bottom of the jar. A vertical carbon rod fits snugly into the tapered hole in the carbon plate, and extends through the cover forming the positive pole. The depolarizer, being heavier than the electrolyte, remains at the bottom of the jar, and the two liquids are thus kept separate. This depolarizer is placed on the market in the form of crystals, known as sulpho-chromic salt, made by the action of sulphuric acid upon chromic acid. When dissolved, its action is similar to that of the chromic acid solution. After the cell has been set up with everything else in place, the crystals are introduced into the solution, near the bottom of the jar, through the vertical glass tube shown, and slowly dissolve and diffuse over the surface of the carbon plate. When the cell current weakens a few tablespoonfuls of the salt introduced through the tube will restore the current to its normal value. The cell should remain undisturbed to prevent the solution from mixing. Its electromotive force is from 1.9 to 2 volts, and the 6 in. × 8 in. size has an internal resistance of about .5 ohm. Since the depolarizer is quite effective, the cell may be used on open or closed circuit work.] The copper sulphate solution, being the heavier of the two, rests at the bottom of the battery jar, while the dilute sulphuric acid remains at the top. To suit this arrangement the copper and zinc elements are located as shown, the copper elements being at the bottom, and the zinc element, shaped like a crow’s foot (hence the name “crowfoot cell”) is suspended at the top. The absence of the porous pot decreases the internal resistance, but the electromotive force is the same as in the ordinary type of Daniell cell.