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Leclanché cell

Georges Leclanché invented and patented in 1866 his battery, the Leclanché cell. It contained a conducting solution (electrolyte) of ammonium chloride, a cathode (positive terminal) of carbon, a depolarizer of manganese dioxide, and an anode (negative terminal) of zinc. The Leclanche battery was essentially a self-contained version of an earth battery, and fairly copied its design.^[1]

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1 Construction
2 Chemistry
3 Usage
4 See also
5 References
6 Sources

Construction

The original form of the cell used a porous pot. This gave it a relatively high internal resistance and various modifications were made to reduce it. These included the "Agglomerate block cell" and the "Sack cell".

Porous pot cell

In Leclanché's original cell the depolarizer, which consisted of crushed manganese dioxide, was packed into a porous pot, and a carbon rod was inserted to act as the cathode. The anode, which was a zinc rod, was then immersed along with the pot in a solution of ammonium chloride. The liquid solution acted as the electrolyte, permeating through the porous pot to make contact with the cathode.

Agglomerate block cell

In 1871 Leclanché dispensed with the porous pot and replaced it with a pair of "agglomerate blocks", attached to the carbon plate by rubber bands. These blocks were made by mixing the manganese dioxide with binding agents and pressing the mixture into moulds.

Sack cell

In this cell the porous pot was replaced by a wrapping of canvas or sacking. In addition, the zinc rod was replaced by a zinc cylinder to give a larger surface area. It had a lower internal resistance than either of the above.

Chemistry

The chemical process which produces electricity in a Leclanché cell begins when zinc atoms on the surface of the anode oxidize, ie they give up both their electrons to become positively-charged ions. As the zinc ions move away from the anode, leaving their electrons on its surface, the anode becomes more negatively charged than the cathode. When the cell is connected in an external electrical circuit, the excess electrons on the zinc anode flow through the circuit to the carbon rod, the movement of electrons forming an electrical current.

When the electrons enter the rod, they combine with molecules of manganese dioxide and molecules of water, which react with each other to produce manganese oxide and negatively charged hydroxide ions. This is accompanied by a secondary reaction in which the negative hydroxide ions react with positive ammonium ions in the ammonium chloride electrolyte to produce molecules of ammonia and water.

$\mathrm{Zn(s)} + 2 \mathrm{MnO_2(s)} + 2 \mathrm{NH_4Cl(aq)} \rightarrow \mathrm{ZnCl_2} + \mathrm{Mn_2O_3(s)} + 2 \mathrm{NH_3(aq)} + \mathrm{H_2O}$

Usage

The electromotive force (emf) produced by a Leclanche cell is typically around 1.5 volts with a resistance of several ohms where a porous pot is used. It saw extensive usage in telegraphy, signalling, electric bells and similar applications where intermittent current was required and it was desirable that a battery should require little maintenance.

The Leclanché battery (or wet cell as it was referred to) was the forerunner of the modern dry cell zinc-carbon battery.

References

^ The Electrical Review, 1892. Page 68.

Sources

Practical Electricity by W. E. Ayrton and T. Mather, published by Cassell and Company, London, 1911, pp 188-193

Category: Electric batteries

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Leclanché_cell". A list of authors is available in Wikipedia.