The Battery Cell.
The battery cell most used in electric bell work is the Leclanche, or some modification of it.
Fig. 2
The Leclanche battery cell is shown in [Fig. 2], where J is a glass jar, Z a rod of zinc, and P a jar of porous earthenware containing a carbon rod surrounded by powdered carbon and peroxide of manganese.
In setting up this cell about four ounces of sal ammoniac (chloride of ammonia) are put into the jar and enough water added to come about half way up the jar.
The porous jar P and the zinc Z are then inserted, and the cell is ready for use in a few minutes after the liquid has soaked through the earthenware into the carbon-manganese mixture. Water is often poured into the porous jar through holes in its top to hasten this wetting.
Wires are clamped by nuts or set-screws to the negative terminal on the zinc or the positive terminal on the carbon, it generally not being of consequence which terminal is attached to either wire of the circuit.
A battery cell could be constructed without the manganese, using simply a plate of carbon and a rod of zinc, but hydrogen gas would be generated on the carbon plate when the cell was working and would stop the current flowing.
This is called polarization, and peroxide of manganese is a de-polarizer, because it combines with this hydrogen gas almost as fast as it is generated, and prevents, to a great extent, the polarization.
But it does not stop it entirely, as will be seen if the Leclanche cell is kept working above its capacity. Then the hydrogen is generated too fast for the manganese to destroy it, and the cell ceases to work. In this case a rest will often restore the cell to its former power.
Cells which have been almost unable to make a bell give even a single tap have been found good again when allowed to remain at rest over night.
In setting up a battery cell no liquid should be splashed on the brass terminals or corrosion will take place. Every metal surface where connection is made to allow electric current to pass must be clean and bright, and all screws, or nuts, holding wires must be screwed up tight so that the wires are firmly clamped.
Loose or dirty connections are the cause of probably eight out of every ten troubles affecting bells and batteries.
When the fluid in a Leclanche cell becomes milky, more sal ammoniac must be added. Or, better still, throw out the old solution, wash the porous jar thoroughly in clean water, scrape the zinc bright, and half fill the cell with fresh solution.
The zinc wearing away rapidly or becoming covered with crystals, and a strong smell of ammonia, show generally that the cell is being worked too hard, or that the current is leaking where it should not.
A zinc rod in a cell working the average door bell should last for six months, the porous jar for a year.