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Table of Contents.
¶ U.S. GOVERNMENT PRINTING OFFICE; 1945  618779
Chapter 6 OHM'S
LAW ITS HISTORY
During the late 1700's and early 1800's, three great electrical
discoveries were made. An Italian, named Volta, discovered haw to. produce
an emf from a primary cell. He gave his name to. the measuring unit
of electromotive force  the VOLT. Ampere, a Frenchman, measured current
flaw and gave his name to. the measuring unit of current  the AMPERE.
A German, named Ohm, measured the resistance of circuits and conductors
and gave his name to the resistance measuring unit  the OHM. Ohm did
more than experiment with resistance he connected his awn discoveries
with those of Volta and Ampere. The result was OHM'S LAW. Make sure
you understand each one of the three quantities in electricity  they
make up Ohm's Law. On the following page is a table of these important
quantities, their symbols, their units, their abbreviations, and their
effects an a circuit.
Quantity 
Symbol 
Unit of Measure 
Abbreviation of Unit 
Effects in a circuit 
EMF or potential or voltage 
E 
the volt 
v. 
Force which makes current flow
through a circuit 
Resistance 
R 
the ohm 
Ω 
The friction or opposition to the
flow of current offered by the conductors and electrical devices
in a circuit 
Current 
I 
the ampere 
a. or amp. 
The flow of electrons through a
circuit Four effects  (1) heat (2) light (3) chemical (4) magnetic 
WHAT IS OHM'S LAW?
You know that increasing the potential ,will INCREASE the current.
Likewise, increasing the resistance will DECREASE the current. Ohm's
law is this relationship of emf, current, and resistance expressed in
mathematical terms.
It says 
That is, the current, I
(in amps) equals the emf, E (in volts) divided by the resistance R,
(in ohms).
Figure 31 shows a simple electrical circuit 
generator, load, and connecting wires. In this case, the load is a lamp,
but ANY electrical appliance is a LOAD. Notice the ammeter connected
to read' the current and the voltmeter connected to read the emf of
the generator. If the resistance of this circuit is 2 ohms and the emf
read on the voltmeter is 12 volts, then 
I=E/R=12/2=6amps.
Figure 31.  Simple circuit, voltage constant.
The ammeter will read 6 amperes. Which means that the load draws
6 amperes.
Figure 32.  Effect of voltage on current,
R is constant.
Imagine that a battery of cells is used, instead o£ a generator,
as an emf source. The circuit. would look like figure 32.
Each
cell produces 2 volts. Notice that connecting the line atA, B, C, or
D, CHANGES the number of cells included in the' circuit. This will give
you voltages of 2, 4, 6, and 8 volts. The table at the. left of the
diagram gives the current flowing for each voltage. For each value of
voltage, the current is calculated by Ohm's law. Remember, current is
DIRECTLY proportional to voltage.
Figure 33.  Effect of resistance on current,
E is constant.
CONCLUSION 
In any electrical circuit, IF YOU HOLD THE RESISTANCE
CONSTANT AND INCREASE THE VOLTAGE, THE CURRENT INCREASES IN PROPORTION
TO THE INCREASE IN VOLTAGE. IF YOU HOLD THE RESISTANCE CONSTANT AND
DECREASE THE VOLTAGE, THE CURRENT DECREASES IN PROPORTION TO. THE DECREASE
IN VOLTAGE.
Take a look at figure 33. In this circuit, the voltage
remains constant, but the resistance of the load is 2,4, 6, or 8 ohms
depending on which tap, A, B, C, or D is connected. In the table at
the left of the figure, the current flow is given for each connection.
Notice that current is INVERSELY proportional to resistance.
CONCLUSION 
In any electrical circuit, IF YOU HOLD THE VOLTAGE
CONSTANT AND INCREASE THE RESISTANCE, THE CURRENT DECREASES IN PROPORTION
TO THE INCREASE IN RESISTANCE. IF YOU HOLD THE VOLTAGE CONSTANT AND
DECREASE THE RESISTANCE, THE CURRENT INCREASES IN PROPORTION TO THE
DECREASE IN RESISTANCE.
Reviewing the tables in figures 32 and
33, you will find that any number in the current column I, can be obtained
by dividing the voltage by the resistance.
I=E/R
Any number in the resistance column R, can be found by dividing
the voltage by the current.
R=E/I
Furthermore, any number in the voltage column, E, can be obtained
by multiplying the current and the resistance.
E=IR
From your knowledge of mathematics, you recognize that these three
equations are variations of one formula.
I=E/R, R=E/I, and E=IR
If you know any two of these quantities in a circuit, or in any
part of a circuit, you can calculate the other quantity by applying
the proper equation.
EXAMPLES
1. A vacuum tube filament has a resistance of 12 ohms when connected
to a 6volt battery. What is the current in the filament?
I=E/R=6/12=1/2 amp.
2. An ignition coil draws 8 amperes at 6 volts. What is the resistance
of the coil?
R=E/I=6/8=3/4 ohm.
3. A starter motor has a resistance of 0.04 ohm and draws 150 amperes
at starting. What is the voltage applied to this motor?
E=IR=0.04x150=6 volts.
REMEMBER THESE POINTS 
1. The strength of the electrical
CURRENT, or amperage, depends on the RESISTANCE of the circuit AND the
VOLTAGE applied to the circuit. Ohm's law will tell you how much current
is flowing.
2. The RESISTANCE does not depend on either current
or voltage. The character of the conducting path  wires and load 
determine the resistance. You DO NOT change resistance by changing current
or voltage. Ohm's law will tell you how much resistance is contained
in the circuit.
3. The emf of a circuit does NOT depend on either
CURRENT or RESISTANCE. The emf is determined entirely by the generator
or battery supplying the circuit. Ohm's law will tell you how much voltage
is required for a given current through a given resistance.
Chapter 6 Quiz
(click
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