Here is the "Electricity  Basic Navy Training Courses"
(NAVPERS 10622) 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
copyright. See
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 how 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 own 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 
Abbrev. 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  I=E/R
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 
Figure 31.  Simple circuit, voltage constant.
Figure 32.  Effect of voltage on current, R is constant.
I=E/R=12/2=6amps.
The ammeter will read 6 amperes. Which means that the load draws 6 amperes.
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 Point 
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 here)
Posted April 24, 2024 (updated from original post
on March 15, 2014)
