When the zinc, Zn, is acted on by the ammonium chloride, 2NH4Cl, the zinc seizes the chlorine and forms with it zinc chloride, ZnCl2, while the ammonium, 2NH4, is liberated. But this ammonium, 2NH4, does not escape. Being electro-positive, it is impelled towards the negative plate, and in its passage thereto meets with another molecule of ammonium chloride, from which it displaces the ammonium, in this wise: 2NH4 + 2NH4Cl = 2NH4Cl + 2NH4; in other words, this electro-positive ammonium is able, by virtue of its electrical charge, to displace the ammonium from the combined chloride. In so doing, it sets the liberated ammonium in an electro-positive condition, as it was itself, losing at the same time its electrical charge. This interchange of molecules goes on (as we saw in the case of the Daniell's cell, [§ 24]) until the surface of the carbon is reached. Here, as there is no more ammonium chloride to decompose, the ammonium 2NH4 immediately splits up into ammonia
2NH3 and free hydrogen H2. The ammonia escapes, and may be detected by its smell; while the hydrogen H2, finding itself in contact with the oxide of manganese, 2MnO2, seizes one atom of its oxygen, O, becoming thereby converted into water H2O; while the manganese dioxide, 2MnO2, by losing one atom of oxygen, is reduced to the form of a lower oxide of manganese, known as manganese sesquioxide, Mn2O3. Expressed in symbols, this action may be formulated as below:—
In the zinc compartment—
Zn + 2NH4Cl = ZnCl2 + 2NH3 + H2
In the peroxide of manganese compartment—
H2 + 2MnO2 = Mn2O3 + H2O.
Ammonia gas therefore slowly escapes while this battery is in action, and this corrodes all the brass work with which it comes into contact, producing a bluish green verdigris. If there be not sufficient ammonium chloride in solution, the water alone acts on the zinc: zinc oxide is produced, which renders the solution milky. Should this be the case, more sal ammoniac must be added. It is found that for every 50 grains of zinc consumed in this battery, about 82 grains of sal ammoniac and 124 grains of manganese dioxide are needed to neutralize the hydrogen set free. It is essential for the efficient working of this battery that both the manganese dioxide and the carbon should be free from powder, otherwise it will cake together, prevent the passage of the liquid, and present a much smaller surface to the electricity, than if in a granular form. For this reason, that manganese dioxide should be preferred which is known as the
"needle" form, and both this and the carbon should be sifted to remove dust.
§ 28. In the admirable series of papers on electric bell fitting which was published in the English Mechanic, Mr. F. C. Allsop, speaking of the Leclanché cell, says:—"A severe and prolonged test, extending over many years, has proved that for general electric bell work the Leclanché has no equal; though, in large hotels, etc., where the work is likely to be very heavy, it may, perhaps, be preferable to employ a form of the Fuller bichromate battery. It is very important that the battery employed should be a thoroughly reliable one and set up in a proper manner, as a failure in the battery causes a breakdown in the communication throughout the whole building, whilst the failure of a push or wire only affects that portion of the building in which the push or wire is fixed. A common fault is that of putting in (with a view to economy) only just enough cells (when first set up) to do the necessary work. This is false economy, as when the cells are but slightly exhausted the battery power becomes insufficient; whereas, if another cell or two had been added, the battery would have run a much longer time without renewal, owing to the fact that each cell could have been reduced to a lower state of exhaustion, yet still the battery would have furnished the necessary power; and the writer has always found that the extra expense of the surplus cells is fully repaid by the increased length of time the battery runs without renewal."