§ 26. There are many other bodies which can be used in batteries to absorb the hydrogen set free. Of several of these we need only take a passing notice, as the
batteries furnished by their use are unfit for electric bell work. Of these we may mention nitric acid, which readily parts with a portion of the oxygen ([§ 18]) and reconverts the free hydrogen into water. This acid is used as the "depolarizer"[9] in the "Grove" and in the "Bunsen" cell. Another very energetic "depolariser" is chromic acid, either in solution, in dilute sulphuric acid, or in the form of potassic dichromate (bichromate of potash: bichrome). As one form of chromic cell has found favour with some bell-fitters, we shall study its peculiarities farther on.
Another class of bodies which readily part with their oxygen, and thus act as depolarisers, are the oxides of lead and manganese. This latter oxide forms the basis of one of the most useful cells for electric bell work, namely: the one known as the "Leclanché." As the battery has been, and will probably remain, long a favourite, the next paragraph will be devoted to its consideration.
§ 27. The Leclanché cell, in its original form, consists in a rod or block of gas carbon (retort scurf: graphite) standing in an upright porous pot. Around this, so as to reach nearly to the top of the porous cell, is tightly packed a mixture of little lumps of graphite and black oxide of manganese (manganic dioxide: black wad), the porous cell itself being placed in an outer containing vessel, which usually takes the form of a square glass bottle. A zinc rod stands in one corner of the bottle,
and is prevented from coming into actual contact with the porous cell by having an indiarubber ring slipped over its upper and lower extremities. The glass containing vessel is then filled to about two-thirds of its height with a solution of ammonium chloride (sal ammoniac) in water, of the strength of about 2 oz. of the salt to each pint of water. This soon permeates the porous cell and reaches the mixture inside. The general appearance of the Leclanché cell is well shown at [Fig. 8].
Fig. 8.
In order to ensure a large surface of contact for the terminal of the carbon rod or plate, it is customary to cast a leaden cap on the top thereof; and, as the porosity
of the graphite, or carbon, is very apt to allow the fluid in the battery to creep up to and corrode the terminal, and thus oppose resistance to the passage of electricity, the upper end of the carbon, before the lead cap is cast on, is soaked for some time in melted paraffin wax, at a temperature of 110° Centigrade: that is somewhat hotter than boiling water heat. This, if left on the outside, would prevent the passage of electricity almost entirely; so lateral holes are drilled into the carbon before the cap is finally cast on. The action that takes place in the Leclanché cell may be summarised as follows:—