CHAPTER I

_The Battery_

=The Battery Cell.= The battery cell most used in electric bell work is the Leclanche, or some modification of it.

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.

=The Dry Cell.= The Leclanche cell being a cell with much free liquid is liable to dry up if not watched. The dry cell (Fig. 3) is a modern form of the Leclanche where the liquid is held by an absorbent material, such as blotting paper, or plaster.

A typical dry cell[A] is shown in the figure. An outside case of zinc is lined with blotting paper dampened with chloride of zinc and sal ammoniac. A carbon rod is then inserted in the centre and packed around with carbon dust and peroxide of manganese. The latter mixture is also somewhat dampened.

[A] For full description of this class of battery see No. 3 Book on “Dry Batteries.”

Molten wax, or a suitable composition, is then poured on top of the contents of the cell to seal it up and prevent the evaporation of the fluid. A terminal on the carbon rod and another on the zinc case complete the cell.

The voltage of both the Leclanche and the dry cell is about 1.45, when it goes below this it indicates that the cell is worked out.

The two cells described are known as open-circuit cells and are only intended for intermittent working.

When a current is needed for a long period at a time a closed circuit cell should be used, such as the gravity Daniell cell.

=The Gravity Daniell Cell.= The gravity cell, Fig. 4, has a zinc block _Z_ suspended from the side of the jar and a number of copper leaves _C_ standing on edge at the bottom. A quantity of bluestone (sulphate of copper) is poured over the copper leaves and the jar filled with water.

During the working of this cell, copper is deposited on the copper plate, and sulphate of zinc formed at the zinc. To hasten the action a small quantity of zinc sulphate can be added to the solution when setting up the cell.

The name of this cell comes from the fact that the copper solution being heavier remains at the bottom of the jar. If the cell is not worked enough, all the solution will become blue and the zinc will blacken. If very dirty from this cause, remove the zinc, scrape and wash it thoroughly. Throw out all the solution, add new sulphate and water and replacing the zinc, then put the cell on short circuit by connecting the copper and zinc together for a few hours.

=E. M. F.= The e. m. f. of a gravity cell is within a fraction of one volt, its current nearly one-half ampere.

Warmth makes it give a greater current; on no account let a gravity cell freeze.

=Resistance of a Cell.= The fluids in a cell do not conduct electricity as well as copper does; they offer more resistance and thus reduce the current output.

The internal resistance of a cell may be lowered by using large zinc plates curled around the porous pot.

The Samson cell has a large zinc plate bent in the form of a cylinder, the carbon-manganese combination standing in the centre of it.

The dry cell also has a large zinc, the internal resistance being thus much lowered, the current output is increased. This is by reason of Ohm’s law, which teaches that to increase the current flow, either the voltage of the battery must be increased, or the resistance decreased.

But increased current means lessened life; there is only just so much energy in a cell mainly dependent on the quantity of chemicals.

=Grouping of Cells.= Cells may be grouped in a battery to get increased voltage, or increased amperage. When connected for the former, they are in series, the carbon of one is connected to the zinc of the next, and so on.

If all the carbons are connected together and all the zincs, they are in multiple, and will give the same voltage as of one cell but the combined amperage of all.

In ordinary bell work the series is the general connection, the higher the resistance of the circuit, or the longer the wires, the more voltage is required.