Here is the "Electricity - Basic Navy Training Courses"
in its entirety. It should provide one of the Internet's best resources for people seeking
a basic electricity course - complete with examples worked out. See
Table of Contents. • U.S. Government Printing Office; 1945 - 618779
Chapter 5: EMF - What It Is
(VERY IMPORTANT NOTE: Current flow here is defined as from negative to positive, which
is opposite of today's convention. Modern convention of positive-to-negative current
flow, with negative-to-positive electron flow requires a "right-hand rule." See
page on RF Cafe).
You have learned that potential difference causes electrons to flow through a conductor.
But-you'll have to know about another "electron-mover." Because it is an "electron-moving-force,"
scientists have named it ELECTROMOTIVE FORCE (emf). Mechanical force is usually measured
in pounds, but emf is measured in VOLTS. Just as pounds of force make water flow through
a pipe, so emf makes current flow through a conductor.
The three terms - POTENTIAL DIFFERENCE, ELECTROMOTIVE FORCE and VOLTAGE - are often
used interchangeably. You will hear electricians say, "VOLTAGE of the generator"; or
"EMF of the generator" and "VOLTAGE of the circuit" or "POTENTIAL of the circuit." However,
note the technical distinctions in these terms. To be ABSOLUTELY CORRECT, EMF should
be applied only to the force produced by a generator or battery. Example - "The emf of
the generator is 120 volts." POTENTIAL or POTENTIAL DIFFERENCE applies to a total circuit
or a part of a circuit. Example - "The potential difference or drop (p.d.) is 63 volts."
The term, VOLTAGE, applies to the number of volts concerned in either case. Example--"The
lamp has a voltage of 120 volts."
Electricians, and sometimes books, confuse these three terms. Don't let it bother
you. Just REMEMBER that to all practical purposes, emf, potential, and voltage mean the
same thing-THE FORCE THAT MAKES CURRENT FLOW.
WHERE IT COMES FROM
When a scientist studies a moving automobile, this is what he sees –
First, a gasoline tank full of chemical energy. Second, an engine which burns this
gasoline and uses the heat energy released to turn a crankshaft. Third, the mechanical
energy of the turning crank-shaft transferred as a force 'on the wheels which move the
The automobile engine, then, is simply a machine which converts chemical energy into
mechanical energy. A steam engine is similar. It takes the chemical and heat energy out
of coal or oil and converts it to mechanical energy in the form of force on moving parts.
No matter what kind of engine or motor you select, you will find that each of them converts
one kind of energy into another kind and then transfers the energy to a force which does
Electrical energy - emf - can be produced by the conversion of four kinds of energy
- mechanical energy, chemical energy, frictional energy, and heat energy. To change these
forms of energy into emf requires "engines" just as the automobile requires an engine
to convert chemical energy into mechanical energy. These electrical "engines" are the
batteries and generators of your circuits.
EMF FROM MECHANICAL ENERGY - THE GENERATOR
The GENERATOR is the "engine" which converts mechanical energy into electrical energy.
It is the most economical and by far the most common source of emf.
Figure 25. - Ship's power - oil to electricity.
Generators consist of two parts - a stationary FRAME and a rotating ARMATURE. The
armature is connected to a source of mechanical energy, called a PRIME MOVER - usually
a turbine, or a gasoline or diesel engine. The prime mover furnishes the energy which
rotates the armature. Then the armature, by a process to be explained later, converts
the mechanical energy to electrical energy. Figure 25 is a representation of a ship's
power plant. Notice the different forms of energy as each machine makes a conversion.
Finally, at the generator, the energy is in the form of electricity ready to be sent
out on the ship's wires to be used to run motors, light lights, power radios, and heat
The exact mechanism by which a generator converts mechanical energy to electrical
energy is very complex. The generator is treated in detail in a later chapter of this
book. The important idea to remember here is that GENERATORS FURNISH A CONTINUOUS EMF
TO A CIRCUIT.
EMF FROM CHEMICAL ENERGY - THE BATTERY
Much energy is stored by nature in chemical compounds (combinations of elements).
Coal, wood, and oil have tremendous stores of energy which are released as heat when
these compounds are burned. Oxygen and hydrogen have so much energy that they explode
when they combine. The electrician is interested in these - substances only because he
can get some of this energy as emf.
Releasing electrical energy from chemical energy is surprisingly simple. If two dissimilar
metals - copper and zinc, for example - are placed in certain chemical solutions, an
emf results. This is the principle employed in all CELLS and BATTERIES - a battery is
simply TWO OR MORE cells connected together.
HOW A CELL WORKS
When two or more atoms of different elements combine, they produce a molecule of a
COMPOUND. For example, atoms of the elements carbon and oxygen combine to form molecules
of carbon dioxide. Carbon dioxide is a COMPOUND, consisting of a combined form of the
elements carbon and oxygen.
When a compound dissolves in certain substances - notably water - it breaks up into
CHARGED PARTICLES. These charged particles are called IONS. Ions are NOT the same as
atoms-ions are charged and atoms are not. You will remember that atoms contain an equal
number of protons and electrons and therefore are neutral. But an ion of a dissolved
compound either loses or gains one or more electrons. If you were to dissolve one molecule
of sodium chloride - common table salt - in water, it would split into a sodium ion and
a chloride ion. But the chloride ion holds on to one of the sodium ion's electrons. This
gives the chloride ion a negative charge and the sodium ion a positive charge. It has
been proved experimentally that a solution containing ions will conduct an electric current.
The ions seem to "ferry" the current through the solution. This should explain to you
why salt water is so likely to produce short circuits aboard ships. Because compounds
that form ions in solution will conduct electric currents, they are called ELECTROLYTES.
Figure 26. - Voltaic cell.
All this leads you to an understanding of the workings of a cell. If any two dissimilar
metal plates are placed in an electrolyte, the ions will develop an emf at the plates.
If the dissimilar plates are then connected by means of a conductor outside the solution,
the emf will force a current through this conductor. This is called a VOLTAIC cell (after
Volta, an early Italian experimenter).
Here's an example of how the Voltaic cell works. Immerse a copper plate and a zinc
plate in a solution of ammonium chloride, as in figure 26. The positive ammonium ions
pick up electrons from the copper plate. This reduces the number of electrons on the
COPPER PLATE and gives it a POSITIVE charge. The chloride ions give up their excess electrons
to the zinc plate. This gives the ZINC plate, an excess of electrons and, therefore,
a NEGATIVE charge. Notice that you have two charged plates - one positive and one negative.
If a wire is connected to the two plates, the potential difference between its two ends
will cause a current to flow.
A number of changes occur in both the plates and electrolyte as the current flows.
The most important change occurs at the zinc plate. Electrons are constantly being lost
by the zinc plate as the current flows, and the zinc atom changes to a zinc ion - which
dissolves in the solution. In short, the zinc is EATEN A WAY by the action. When the
zinc is completely ionized (dissolved) - the cell's emf ceases. Because the action of
the cell uses up a primary part, the cell is called a PRIMARY cell. Such a cell cannot
The most common primary cell is one you have seen many times - the "dry cell." Figure
27 shows a cross section view of a dry cell. The two plates, called ELECTRODES, are zinc
and carbon. Notice how the zinc electrode is shaped to form a cylindrical can. Thus,
the electrode serves as the cell case. The electrolyte is ammonium chloride dissolved
in water and mixed with paste. The paste is merely to prevent the electrolyte from spilling.
Figure 27. - Cross section of a dry cell.
Figure 28. - Lead-acid storage battery.
Figure 29. - Charging and discharging the lead storage battery.
Figure 30. - Thermocouple construction.
The chloride ions lose electrons to the zinc plate, giving it a negative charge. The
ammonium ions pick up electrons from the carbon rod giving this electrode a positive
charge. In this type of cell about 1.5 volts of emf are developed. Once the outside circuit
is completed, the zinc begins to dissolve. Since this is a primary cell, the action ceases
when all the zinc is used up. Usually the paste electrolyte will leak from a "dead" cell
because the zinc container is eaten away. To avoid messy leakage, dry cells should be
removed from flashlights, lanterns, and radios when the gear is stowed or the batteries
are worn out.
There are many kinds of primary cells - differing from each other in the materials
used for electrodes and electrolytes. The amount of emf produced by each depends on the
materials composing the electrodes and electrolyte.
Primary cells are useful for only a short time. Their chemical energy is used up and
they must be discarded. SECONDARY cells also lose their chemical energy, BUT, they can
be restored by passing an electric current through them. The lead storage cell is a common
example of the secondary cell. It is used in battery form (two or more cells connected
together) in automobiles, motor launches and submarines.
The lead storage cell has electrodes of lead and lead oxide immersed in a solution
of sulphuric acid. The complete construction is shown in figure 28. During use, both
plates are changed to lead sulphate, and much of the sulphuric acid is converted to water.
These changes mean that the chemical energy of the cell has been converted to electrical
energy. The cell is DISCHARGED. If a current from another source is passed through this
cell IN THE OPPOSITE DIRECTION TO THAT OF DISCHARGE, THE CHEMICAL ENERGY IS RESTORED.
That is, the plates again become lead and lead oxide and the water is changed back to
sulphuric acid. The cell is now CHARGED and ready to deliver an emf. Figure 29 shows
the difference between a charged and discharged cell. With proper care, a secondary cell
can be charged and discharged many many times.
EMF FROM FRICTION
Friction between two unlike substances results in a potential difference between those
substances. This potential difference is an emf which will move electrons toward the
lower potential. You are familiar with this kind of emf production-it is the STATIC CHARGE.
Although, this was the earliest discovered method of producing an emf, there is relatively
little practical use for static electricity. Most of it is wasted as static discharges.
EMF FROM HEAT
When two unlike metals, such as platinum and rhodium, are bound together and heated,
they produce an emf. This arrangement of two metals is called a THERMOCOUPLE The strength
of the emf produced by any thermo-couple is proportional to the temperature. Thermo-couples
are used in steel furnaces, boiler flues, stacks, and molten metals to measure extremely
high temperatures. Notice in figure 30 that the two wires are twisted together at .one
end and then welded. This is the end to be heated. The balance of the wires are insulated
by porcelain beads to prevent a short circuit.
A very sensitive voltmeter is used to measure the emf produced by the thermo-couple.
In the example used-a platinum and rhodium thermocouple-a temperature of 920 °F. will
produce 0.003672 volt but a temperature of 1980 °F. will produce 0.010534 volt. Notice
that increasing the temperature increases the voltage of the emf. When a thermo-couple
is used to measure temperature, the voltmeter is calibrated IN DEGREES INSTEAD OF IN
VOLTS. Thus the voltmeter reads directly the temperature of the thermo-couple.
The thermocouple, used to measure temperature, is the only practical use made of emf
produced by heat.
Chapter 5 Quiz