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CHAPTER 2
BATTERIES
ELECTROMOTIVE FORCE

ALL FORCES that tend to keep electrons moving through a conductor are called ELECTROMOTIVE FORCES. That should not be difficult to remember if you think of it as ELECTRON-MOVING-FORCE, or just EMF.

PRIMARY CELL

BATTERIES, or CELLS, as single units of a battery are called, are extremely common. If you have silver and gold fillings in your teeth, you are carrying a simple cell in your mouth.

Why ? A SIMPLE CELL is formed whenever you have TWO DIFFERENT METALS in an ELECTROLYTE.

GOLD is one metal; SILVER is another; and SALIVA is an electrolyte. An electrolyte is any liquid, such as an acid, saltwater or an alkali, that will CONDUCT ELECTRICITY.

A simple cell, sometimes called a primary cell, will continue to deliver current until ONE OF THE METALS has been EATEN AWAY, or until the ELECTROLYTE IS EVAPORATED.

 
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The cell, once dead, CANNOT BE RECHARGED. The only way to bring it back to life is to put in new plates and replace the electrolyte.

HOW A PRIMARY CELL WORKS

Most metals have a tendency to give away ELECTRONS and become POSITIVELY charged. Some metals, like copper and silver, have a much stronger tendency to give away their electrons than do zinc and iron. Therefore if you place a strip of COPPER and another of ZINC in an electrolyte such as ammonium chloride (see figure 14),

A simple cell.
Figure 14.-A simple cell.
the COPPER will give up electrons and become MORE positive. In the external circuit, electrons will flow away from the zinc, through the resistor, and onto the copper plate.

The chemical action going on inside the cells is too complicated for a discussion at this time, but here is just a hint of what happens. The ammonium chloride breaks into positively and negatively charged particles called

 
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IONS. These IONS act as FERRY BOATS to carry the electrons from the copper plate to the zinc plate. It is this CHEMICAL ACTION that produces the emf.

The COMBINATIONS of metal used play a big part in the action of the cell. The 10 metals and carbon given below are arranged in order, with gold, the most positive, on top.

Gold
Carbon (not a metal)
Mercury
Silver
Copper
Lead
Tin
Nickel
Iron
Zinc
Aluminum

The second most positive is carbon, next mercury, and so on down the list. In short any metal will be POSITIVE to any metal that appears BELOW IT. Now, imagine any two metals in a simple cell and connected by an external circuit. Electrons will flow through the external circuit FROM THE LOWER METAL TO THE HIGHER METAL.

The FARTHER APART the two metals appear in the table, the larger will be the difference in their potential. If gold and aluminum are used, the emf will be 2.69 volts. With carbon and zinc, the emf will be 1.8 volts; while with copper and zinc it is only 1.1 volts.

The output voltage of a cell will never be as great as the two metals used indicate, because the INTERNAL RESISTANCE of the CELL (electrolyte) SUBTRACTS from the potential difference of the plates. As an example, the actual emf of a carbon-zinc cell is only about 1.5 volts instead of 1.8 volts.

DRY CELL

While primary cells can be used with a liquid electrolyte, it is a common practice to mix the electrolyte

 
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with a POWDER, usually manganese-dioxide, to form a paste. The result is a common DRY CELL.

The paste is placed inside a ZINC can, and a CARBON rod inserted into the paste as illustrated in figure 15.

Cross section of a dry cell.
Figure 15.-Cross section of a dry cell.
A heavy paper washer is placed in the bottom of the can to prevent the carbon from touching the zinc. The sawdust, sand, and pitch form a seal to prevent the electrolyte from evaporating.

The dry cell becomes dead when the zinc can has been eaten away, and the electrolyte has evaporated. Dry cells can be brought back to life temporarily by punching holes in the zinc can and then submerging the cell in a pail of water for five or ten minutes. This is only an emergency measure, but it may help you out of a tight spot some time.

 
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SECONDARY CELLS

SECONDARY or STORAGE cells are those that can be RECHARGED. They are used whenever a larger supply of current is needed than can be furnished by dry cells.

The plates of a storage cell are usually made of LEAD, and the positive plates are coated with LEAD PEROXIDE. The electrolyte is SULFURIC ACID.

In figure 16, when the cell is discharging, electrons flow from the negative lead plate through the load to the positive lead-peroxide plate. The lead-peroxide combines with sulfuric acid to form lead sulfate and water. During discharge lead sulfate is deposited on both plates.

When the cell is being charged (figure 16), the current is FORCED to reverse its direction. The lead sulfate is

Charging and discharging of a storage cell.
Figure 16.-Charging and discharging of a storage cell.
changed back to lead peroxide on the positive plate, and to lead on the negative plate. This action returns sulfuric acid to the electrolyte, which increases in strength.

In an automobile cell this process of charging and discharging goes on hundreds of times. When the cell discharges, it supplies current to the lamps, the starter, and

 
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a host of other instruments. But while the cell is charging, a direct current generator forces electrons to flow backwards through the cell. This rebuilds the plates and restores the electrolyte.

The strength of the sulfuric acid is used to indicate whether the cell is charged or discharged. If the HYDROMETER-a battery tester-reads less than 1,100, the cell is almost dead, but when it shows a value greater than 1,350, it is well charged.

CELLS AND BATTERIES

When several individual units such as three dry cells are connected together, they form a BATTERY. A single unit is not a battery but a cell.

Lead-acid storage cell and battery.
Figure 17.-Lead-acid storage cell and battery.
 
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In figure 17 the top drawing shows a cutaway of a storage cell, while the lower drawing shows a three-cell battery.

CELLS IN SERIES AND PARALLEL

Cells are connected together to obtain either INCREASED emf or an INCREASED AVAILABLE SUPPLY OF CURRENT.

Cells in series.
Figure 18.-Cells in series.
Connecting cells in series-that is, positive-to-NEGATIVE, positive-to-negative, and so on-increases the total emf output.

In figure 18 the three cells, each 1.5 volts, are connected in series. The total emf of the combination is 4.5 volts. If four cells are used, the output emf will be-

4 X 1.5 = 6 volts

Cells in parallel.
Figure 19.-Cells in parallel.
 
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Each cell of the storage battery in figure 18 has an emf of 2 volts. The three connected in series will have an output voltage of-

2 x 3 = 6 volts

When you wish to obtain an INCREASED AVAILABLE SUPPLY OF ELECTRONS, you will connect the cells in PARALLEL-that is, connect together all the positive terminals and all negative terminals as indicated in figure 19.

The output voltage of cells in parallel is equal to that of a single cell-but the available current is approximately equal to the current of a single cell TIMES THE NUMBER of cells.

By making proper combinations of series and parallel cell connections, wide varieties of both emf and available current supply can be obtained.

SCHEMATIC SYMBOL FOR CELLS AND BATTERIES

Usually you will see the schematic symbol used to indicate a cell or battery, rather than a pictorial representation . The symbols for a single cell, cells in series, and

-Symbols for cells in series and parallel.
Figure 20.-Symbol for cells in series and parallel.
cells in parallel are given in figure 20. The LONGER LINE is the positive terminal of a cell.
 
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