simple experiments use the dilute acid (App. 14). If for small motors,
use the formula given in App. 15. The zinc should be well amalgamated.
(App. 20.)

APPARATUS 6.

[Illustration: Fig. 6.]

_7. Experimental Cell._ Fig. 6. In some experiments a comparison is made
between cells with large plates and cells with small ones. This form
will be convenient to use where narrow plates are desired. Those shown
are 4 x 1/2 in. They are screwed to the cross-piece, which is 4-1/2 x 1
x 7/8 in. Do not let the screws touch each other. The wires are fastened
under the screw-heads.

APPARATUS 7.

[Illustration: Fig. 7.]

_8. Experimental Two-fluid Cell._ Fig. 7. This cell has a zinc strip, Z,
and copper cylinder, C, for the "elements." The porous cup, P C, is
fully described in App. 11. Z is 5 x 1 in., and should be well
amalgamated (App. 20). (Study reasons for amalgamation.) A zinc rod,
like that shown in Fig. 4, may be used instead of the strip. The copper
cylinder, C, nearly surrounds P C, and is made from a piece of thin
sheet-copper, 6 x 2 in. The narrow strip, or leader, A, is 5 x 1/2 in.
To fasten it to C, punch two small holes in C and A, put short lengths
of stout copper wire through the holes, and hammer them down so that
they will act as rivets, R. C can be hung centrally in the tumbler by
bending A as shown. Y and X are spring binding-posts (App. 42). The
battery wires can be fastened directly to Z and A, as suggested in Fig.
4.

9. Setting up the Cell. Arrange as in Fig. 7, but remove Z from P C.
Pour some of the acid solution of App. 14 into P C until it stands about
2-1/2 in. deep, and at once pour the copper solution of App. 16 in the
tumbler, on the outside of P C, until it stands at the same height as
the liquid in P C. As soon as the liquids have soaked into P C, you can
put Z in place, when the cell will be ready for use. Remove and wash Z,
when you have finished, and if you wish to use this cell occasionally,
remove the liquids and wash P C thoroughly in water. When dry it will be
as good as new. The acid rapidly acts upon Z, so it is better to remove
Z if you wish to leave the experimenting even for a few minutes only.

Put a few crystals of copper sulphate (blue vitriol) in the tumbler
under the copper, to keep the copper solution saturated. (See text-book
for the chemical action in this two-fluid cell.)

APPARATUS 8.

_10. Two-fluid Battery._ Fig. 8. When two or more cells are joined
toget