04.18.08
Lalande Cell
Lalande Cell:–A type of cell, specially adapted to constant-current
work, and sometimes used as a central source of current in very small
common-battery exchanges is the so-called _copper oxide_, or _Lalande
cell_, of which the Edison and the Gordon are types. In all of these
the negatively charged element is of zinc, the positively charged
element a mass of copper oxide, and the electrolyte a solution of
caustic potash in water. In the Edison cell the copper oxide is in the
form of a compressed slab which with its connecting copper support
forms the electrode. In the Gordon and other cells of this type the
copper oxide is contained loosely in a perforated cylinder of sheet
copper. The copper oxide serves not only as an electrode, but also as
a depolarizing agent, the liberated hydrogen in the electrolyte
uniting with the oxygen of the copper oxide to form water, and leaving
free metallic copper.
On open circuit the elements are not attacked, therefore there is no
waste of material while the cell is not in use. This important
feature, and the fact that the internal resistance is low, make this
cell well adapted for all forms of heavy open-circuit work. The fact
that there is no polarizing action within the cell makes it further
adaptable to heavy closed-circuit service.
These cells are intended to be so proportioned that all of their parts
become exhausted at once so that when the cell fails, complete
renewals are necessary. Therefore, there is never a question as to
which of the elements should be renewed
After the elements and solution are in place about one-fourth of an
inch of heavy paraffin oil is poured upon the surface of the solution
in order to prevent evaporation. This cell requires little attention
and will maintain a constant e.m.f. of about two-thirds of a volt
until completely exhausted. It is non-freezable at all ordinary
temperatures. Its low voltage is its principal disadvantage.
_Standard Cell_. Chloride of Silver Cell:–The chloride of silver cell
is largely used as a standard for testing purposes. Its compactness
and portability and its freedom from local action make it particularly
adaptable to use in portable testing outfits where constant
electromotive force and very small currents are required.
[Illustration: Fig. 66. Chloride of Silver Cell]
A cross-section of one form of the cell is shown in Fig. 66. Its
elements are a rod of chemically-pure zinc and a rod of chloride of
silver immersed in a water solution of sal ammoniac. As ordinarily
constructed, the glass jar or tube is usually about 2-1/2 inches long
by 1 inch in diameter. After the solution is poured in and the
elements are in place the glass tube is hermetically sealed with a
plug of paraffin wax.