281. Setting Up the Two-Fluid Cell. [Fig. 87]. Stand the amalgamated zinc rod, Zn, in P C, then place P C in the tumbler, G T; put in the copper plate as shown. Pour diluted acid ([§ 258]) into P C until it stands about 2½ in. deep; then at once pour the copper solution ([§ 283]) into G T, on the outside of P C, until it stands at the same height as the acid in P C. As soon as the liquids have soaked into P C the cell will be ready for use; but it is better to connect it with the galvanoscope at once and note the increase of current during the first few minutes while the liquids soak through P C. A crystal of copper sulphate should be put outside of P C to keep the solution full strength. This is a form of the well-known Daniell cell. [Fig. 87] shows a form of home-made two-fluid cell as described in Apparatus Book. If you have the one furnished, use the rod instead of sheet zinc, and use connectors on the plates.
282. Care of Two-Fluid Cell. This experimental cell should be taken apart when not in use. It should not be left in open circuit, even for half an hour. Even after the plates are removed, copper may be deposited upon P C in case there are any metallic impurities on it. Remove the plates and P C, and thoroughly wash them. The copper solution should be put into a bottle to prevent evaporation; the dilute acid may be thrown away to be replaced by fresh acid for the next experiment.
283. Copper Sulphate Solution is made by adding the blue crystals to water until no more will dissolve—that is, the solution should be "saturated," extra crystals always being left in the stock bottle. An ounce of the crystals to half a tumbler of water will be about right, but a pint or so should be made at a time and be kept bottled to save time.
284. Directions. (A) In case you have access to a pair of scales that weigh to within 0.1 gram, carefully weigh both copper and zinc before proceeding. They should be washed and dried with a cloth. See that there are no drops of mercury upon the zinc that may fall off during the experiment.
(B) Replace the plates in the cell, and connect them with the 15-turn coil of G V, placed N and S. Allow the circuit to remain closed for half an hour, and record the position of the needle every 5 minutes.
(C) Again wash, dry with a cloth without rubbing, and weigh both the zinc and copper. Compare the new weights with those found in (A).
285. Chemical Action in the Two-Fluid Cell. In this form of cell the hydrogen is not allowed to collect upon the copper plate. The action inside of P C is like[113] that explained in [§ 270], hydrogen being set free. As soon as this hydrogen (H2) comes in contact with the copper sulphate (CuSO4), and it begins to do this in the walls of P C, a new chemical reaction takes place. Hydrogen has a stronger attraction for SO4 than Cu has, so it unites with the SO4, forming H2SO4 (sulphuric acid), and this at the same time throws out the Cu bodily. This Cu is then free, instead of hydrogen, to be deposited upon the copper plate. The current is constant, as there is no polarization.
286. Various Galvanic Cells; Open and Closed Circuit Cells. There is no one form of cell that is best for all kinds of work. If momentary currents only are wanted, such as for bells, telephones, etc., in which the cell has plenty of time to rest, open circuit cells are used. These cells polarize, however, when the circuit has to be closed for any length of time. This form of cell is always ready for use, and may not need attention for months at a time. The most common forms of the open circuit cells are the Leclanché ([§ 287]) and dry cell ([§ 288]). Open circuit cells polarize quickly, because the depolarizer ([§ 280]) is slow in destroying the hydrogen.
When a strong current is needed for a considerable time, such as for telegraph lines, motors, etc., a closed circuit cell is necessary. The depolarization must be rapid and constant. The bichromate ([§ 289]) and the Daniell cell ([§ 290]) are very common forms of closed circuit cells. (See, also, Storage Cells.)
287. The Leclanché Cell is an open circuit cell in which carbon and zinc are the plates. The carbon is surrounded with dioxide of manganese, a depolarizer; the two are either packed together in a porous cup, or the latter is compressed into blocks, which are fastened to the carbon. The porous cup stands in a jar containing a solution of sal ammoniac (ammonium chloride), which acts as the exciting fluid, and in which stands a zinc rod. The zinc is not acted upon when the circuit is open. The hydrogen given off by the decomposition of the ammonium chloride is destroyed by the oxygen contained in the manganese dioxide. The E. M. F. is nearly 1.5 volts.