By far the better mode is to rub round the inside and outside of the neck of the jar with tallow, or melted paraffin wax, to the depth of an inch or thereabouts. This effectually prevents creeping and the consequent loss of current. Messrs. Gent, of Leicester, have introduced a very neat modification of the Leclanché cell, with a view to obviate altogether the evils deriving from creeping. This cell is illustrated at [Fig. 12], and the following is the description supplied by the patentees:—"All who have had experience of batteries in which a solution of salts is used are aware of the difficulty experienced in preventing it creeping over the outside of the jar, causing local loss, and oftentimes emptying the jar of its solution. Many devices have been tried to prevent this, but the only effectual one is our patent insulated jar, in which a recess surrounds the top of the jar, this recess being filled with a material to which the salts will not adhere, thus keeping the outside of the

jar perfectly clean. It is specially adapted for use in hot climates, and is the only cell in which jars may touch each other and yet retain their insulations. We confidently recommend a trial of this cell. Its price is but little in excess of the ordinary Leclanché." The battery should be set up in as cool a place as possible, as heat is very conducive to creeping. It is also important that the battery should be placed as near as convenient to the bell.

Sometimes the zincs are seen to become coated with a black substance, or covered with crystals, rapidly wasting away at the same time, although doing little or no work; a strong smell of ammonia being given off at the same time. When this occurs, it points to an electrical leakage, or short circuit, and this, of course, rapidly exhausts the battery. It is of the utmost importance to the effective working of any battery that not the slightest leakage or local action should be allowed to take place. However slight such loss be, it will eventually ruin the battery. This leakage may be taking place in the battery, as a porous cell may be broken, and carbon may be touching the zinc; or out of the battery, along the conducting wires, by one touching the other, or through partial conductivity of a damp wall, a metallic staple, etc., or by creeping. If loss or local action has taken place, it is best, after discovering and repairing the faults (see also testing wires), to replace the old zincs by new ones, which are not costly.

§ 33. There is yet a modification of the Leclanché

which is sometimes used to ring the large bells in hotels, etc., known as the Leclanché reversed, since the zinc is placed in the porous pot, this latter being stood in the centre of the stoneware jar, the space between the two being packed with broken carbon and manganese dioxide. By this means a very much larger negative surface is obtained. In the Grenet cell, the porous cell is replaced by a canvas bag, which is packed full of lumps of graphite and carbon dioxide, a central rod of carbon being used as the electrode. This may be used in out-of-the-way places where porous cells are not readily obtainable, but I cannot recommend them for durability.

§ 34. The only other type of battery which it will be needful to notice in connection with bell work is one in which the depolariser is either chromic acid or a compound of chromic acid with potash or lime. Chromic acid consists of hydrogen united to the metal chromium and oxygen. Potassic dichromate (bichromate of potash: bichrome) contains potassium, chromium, and oxygen. If we represent potassium by K, chromium by Cr, and oxygen by O, we can get a fair idea of its constitution by expressing it as K2Cr2O7, by which it is shown that one molecule of this body contains two atoms of potassium united to two atoms of chromium and seven atoms of oxygen. Bichromate of potash readily parts with its oxygen; and it is upon this, and upon the relatively large amount of oxygen it contains, that its efficiency as a depolariser depends. Unfortunately, bichromate of potash is not very soluble in water; one pint of water will not take up much more than three

ounces of this salt. Hence, though the solution of potassium bichromate is an excellent depolariser as long as it contains any of the salt, it soon becomes exhausted. When bichromate of potash is used in a cell along with sulphuric acid and water, sulphate of potash and chromic acid are formed, thus:—

K2Cr2O7+H2SO4+H2O=K2SO4+2H2CrO4
┗━━━┛ ┗━━┛ ┗━┛ ┗━━┛ ┗━━━┛
1 molecule of bichrome.&1 molecule of sulphuric acid.&1 molecule of water.give1 molecule of sulphate of potash.&2 molecules of chromic acid.

From this we learn that before the potassium bichromate enters into action in the battery, it is resolved into chromic acid. Chromic acid is now prepared cheaply on a large scale, so that potassium bichromate may always be advantageously replaced by chromic acid in these batteries; the more so as chromic acid is extremely soluble in water. In the presence of the hydrogen evolved during the action of the battery ([§ 18]) chromic acid parts with a portion of its oxygen, forming water and sesquioxide of chromium, Cr2O3, and this, finding itself in contact with the sulphuric acid, always used to increase the conductivity of the liquid, forms sulphate of chromium. The action of the hydrogen upon the chromic acid is shown in the following equation:—

2H2CrO4+3H2=5H2O+Cr2O3
┗━━━┛ ┗━┛ ┗━━┛ ┗━━┛
2 molecules of chromic acid.&3 molecules of hydrogen.give5 molecules of water.&1 molecule of chromium sesquioxide.