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),
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.
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
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.
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.
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
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
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.
