The telegraph manual: a complete history and description of the semaphoric, electric and magnetic telegraphs of Europe, Asia, Africa, and America, ancient and modern.

94 VOLTAIC ELECTRICITY.
promises to be a still further improvement. With graphite
conducting plates there is no occasion for the wooden frame.
A single zinc plate, with a binding-screw soldered to it, occupies the central place, instead of the platinized foil, and two
flat pieces of graphite may be clamped on each side; care being
taken to insulate the zinc from the graphite by small strips of
varnished wood. It will be observed that in this disposition of
the apparatus with the graphite, the position of the exciting
zinc in reference to the conducting surfaces is transposed, as
well as the proportions of each to the other being reversed; a
single plate of zinc being placed between two conducting surfaces instead of the conducting surface being in the centre,
with a zinc plate on each side.
Fig. 16 is another form of the Smee cell as practically applied by Mr. Hall of Boston, with great success as to its effiFig. 16. Fig. 17.
ciency and long service. The zinc plates are large, and the
platinized sheet very thin. Fig. 17 is composed of three cells
united by the wires, one connecting with the copper and the
other with the zinc, the two poles of the battery.
The object in every case is to obtain from a given quantity
of the exciting* metal the greatest possible amount of current
electricity, without allowing the power to be wasted in other
ways. The consumption of a given weight of zinc cannot, by
any possible combination, excite more electricity than will decompose a quantity of water equivalent to that which is decomposed by the chemical affinity of the metal for oxygen.
Thus, supposing two grains of water to be decomposed in the
generating cell, and eight grains of zinc to be oxydized, the
electricity generated during the process cannot be more than