ly used in connection with circuits that are continuously
closed, they are called _closed-circuit cells_. The other cells, which
are better adapted for intermittent current, are commonly used on
circuits which stand open most of the time and are closed only
occasionally when their current is desired. For this reason these are
termed _open-circuit cells_.

_Open-Circuit Cells_. LeClanche Cell:--By far the most important
primary cell for telephone work is the so-called LeClanche cell. This
assumes a large variety of forms, but always employs zinc as the
negatively charged element, carbon as the positively charged element,
and a solution of sal ammoniac as the electrolyte. This cell employs a
chemical method of taking care of polarization, the depolarizing agent
being peroxide of manganese, which is closely associated with the
carbon element.

The original form of the LeClanche cell, a form in which it was very
largely used up to within a short time ago, is shown in Fig. 61. In
this the carbon element is placed within a cylindrical jar of porous
clay, the walls of this jar being of such consistency as to allow
moisture slowly to permeate through it. Within this porous cup, as it
is called, a plate or disk of carbon is placed, and around this the
depolarizing agent, consisting of black oxide of manganese. This is
usually mixed with, broken carbon, so as to increase the effective
area of the carbon element in contact with the depolarizing agent, and
also to reduce the total internal resistance of the cell. The zinc
electrode usually consisted merely in a rod of zinc, as shown, with a
suitable terminal at its upper end.

[Illustration: Fig. 61. LeClanche Cell]

The chemical action taking place within the LeClanche cell is,
briefly, as follows: Sal ammoniac is chemically known as chloride of
ammonium and is a combination of chlorine and ammonia. In the action
which is assumed to accompany the passage of current in this cell, the
sal ammoniac is decomposed, the chlorine leaving the ammonia to unite
with an atom of the zinc plate, forming chloride of zinc and setting
free ammonia and hydrogen. The ammonia is immediately dissolved in the
water of the cell, and the hydrogen enters the porous cup and would
speedily polarize the cell by adhering to the carbon plate but for the
fact that it encounters the peroxide of manganese. This material is
exceedingly rich in oxygen and it therefore readily gives up a part of
its oxygen, wh