Chapter I: , for Home-Made Two-Fluid Cells.)

=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 2-1/2 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 that explained in § 270, hydrogen being set free. As soon as this hydrogen (H_{2}) comes in contact with the copper sulphate (CuSO_{4}), and it begins to do this in the walls of P C, a new chemical reaction takes place. Hydrogen has a stronger attraction for SO_{4} than Cu has, so it unites with the SO_{4}, forming H_{2}SO_{4} (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.

=288. Dry Cells= are for open circuit work. Sheet zinc forms
at the same time the active plate and the outside cylinder
or case. A carbon plate acts as the inactive or - plate. The
exciting fluid is kept from spilling by its being absorbed by
one of the various substances used for that purpose.

=289. The Bichromate of Potash Cell= is a very common one
for laboratory use. It gives a strong current, and although
a single fluid cell, it does not readily polarize. Zinc and
carbon plates are used. In the sulphuric acid, which is the
exciting fluid, is dissolved bichromate of potash. This cell
is used for running small motors, induction coils, etc. It
has, with fresh solutions, an E. M. F. of about 2 volts. (See
Apparatus Book, Chapter I., for Home-Made Batteries.)

=290. The Daniell Cell=, of which the two-fluid cell used in Exp.
112 is a form, is noted for its constant current. The E. M. F.
is a little over 1 volt, and it should be kept working through
a resistance when not in regular use; it should not be left in
open circuit. The porous cup keeps the two fluids from mixing,
but it does not stop the current.

=291. The Gravity Cell= is a form of the above. As one of the
fluids is heavier than the other, no porous cup is needed.
Gravity, together with the action of the current, tends to
keep the fluids separated. A copper plate is placed in the
bottom of the jar, and upon this is put the copper sulphate
solution. The zinc plate is supported by the top of the jar and
rests in a solution of zinc sulphate, which is lighter than
the blue solution below. An insulated wire extends from the
copper through the liquids. This cell is used for telegraph and
similar work. (See Apparatus Book for Home-Made Gravity Cell,
its Regulation, etc.)