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, which forms water by combination with the already liberated hydrogen and leaves what is termed a sesquioxide of manganese.  This absorption or combination of the hydrogen prevents immediate polarization, but hydrogen is evolved during the operation of the cell more rapidly than it can combine with[typo was ‘wth’] the oxygen of the manganese, thereby leading to polarization more rapidly than the depolarizer can prevent it when the cell is heavily worked.  When, however, the cell is left with its external circuit open for a time, depolarization ensues by the gradual combination of the hydrogen with the oxygen of the peroxide of manganese, and as a result the cell recuperates and in a short time attains its normal electromotive force.

The electromotive force of this cell when new is about 1.47 volts.  The internal resistance of the cell of the type shown in Fig. 61 is approximately 1 ohm, ordinarily less rather than more.

A more recent form of LeClanche cell is shown in cross-section in Fig. 62.  This uses practically the same materials and has the same chemical action as the old disk LeClanche cell shown in Fig. 61.  It dispenses, however, with the porous cup and instead employs a carbon electrode, which in itself forms a cup for the depolarizing agent.

[Illustration:  Fig. 62.  Carbon Cylinder LeClanche Cell]