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.

The e.m.f. of a cell of this type is 1.03 volts and the external resistance varies with the age of the cell, being about 4 ohms at first.  Care should be taken not to short-circuit these cells, or use them in any but high-resistance circuits, as they have but little energy and become quickly exhausted if compelled to work in low-resistance circuits.