her the combination is called a battery. Fig. 8 shows two
experimental cells joined in series. (Study methods of joining cells.)
For convenience, and to keep them from being easily overturned, a frame
has been made for them. The base, B, is 8 x 4 x 7/8 in. To the back of
this is nailed the upright board, A, 8 x 4-1/2 x 1/2 in. On the top of A
are 3 binding-posts, 1, 2, 3, which consist of metal strips 1-1/4 x 1/2
in. At the lower ends are screws which are connected with the cells, as
shown. Spring binders can be easily slipped on and off the upper ends of
the strips, so that one or two cells can be used at will. Bent strips,
C, are nailed to B, to hold the tumblers firmly in place. This framework
is not necessary, of course, to the proper working of the battery, but
with it you are much less liable to upset the cells.

[Illustration: Fig. 8.]

APPARATUS 9.

_11. Gravity Cell._ Fig. 9. In the two-fluid cell of App. 7 the fluids
were kept apart by the porous cup. The gravity cell is really a
two-fluid cell in which the two liquids are kept separate by the joint
action of the current and the force of gravity. This cell is used for
telegraph lines and for other closed-circuit work.

12. Construction. The zinc and copper, Z and C, Fig. 9, can be purchased
about as cheaply as you can make them. There are many forms of the
zincs, the one shown being called the crow-foot shape. The copper may be
star-shaped, or as shown. If you wish to make C, use thin sheet-copper.
Brush copper, 1-3/4 in. wide, is excellent for the purpose. Use a piece
12 or 15 in. long, and fasten to one end of it a copper wire, W, which
must be covered with paraffined paper, or with rubber or glass tubing,
where it passes up through the zinc sulphate solution and near Z. The
glass jar, J, may be made from a large glass bottle. (See index for
battery jars.)

13. To Set Up the Cell. (A) Place C upon the bottom of J, with W in the
position shown. (B) Put in enough copper sulphate crystals to cover the
bottom of J, but do not try to entirely cover C. At the start 1/2 lb.
will be enough. (C) Pour in clean water until J is half full. (D) In
another vessel dissolve 1 or 2 oz. of zinc sulphate in enough water to
complete filling, J. (E) Hang Z in place (Fig. 9). Z must never touch C.
They should be about 3 in. apart. A wire is attached to Z by the screw,
S, and the hole, H. (F) Pour the zinc sulphate solution into J until it
is within an inch of the top. It s