_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 of the cell or