his 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.

Conventional Symbol. The conventional symbol for a cell, either of
the primary or the secondary type, consists of a long thin line and a
short heavy line side by side and parallel. A battery is represented
by a number of pairs of such lines, as in Fig. 67. The two lines of
each pair are supposed to represent the two electrodes of a cell.
Where any significance is to be placed on the polarity