December 1972 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Here is a brief but
informative introduction to the story behind French physicist
Andre Marie Ampere's
discovery of the eponymously named law that governs the relationships between current
flow and a magnetic field. It appeared in a 1972 issue of Popular
Electronics magazine. As most RF Cafe visitors know, both a steady state and
time-varying current will generate a magnetic field, but only a time-varying magnetic
field can generate a current flow. In less than a week after witnessing
Christian Ørsted's demonstration of a current-carrying wire influencing a compass
needle, Ampere discovered the
Right-Hand Rule of current flow direction based on the direction
of the magnetic field.
By David L. Heiserman
Ampere's Law states that a pair of conductors
carrying electrical currents exert magnetic forces upon one another. Furthermore,
the amount of that force depends upon the amount of current flowing in each conductor,
and the distance and angle between them. Andre Marie Ampere, a French physicist
and mathematician, announced this new law of nature on September 18, 1820. As if
discovering such a law weren't enough, Ampere used it to lay the theoretical foundations
for a whole new branch of electricity and physics called electro-dynamics - and
he did it in just seven years.
Looking back at Ampere's work from our present-day
point of view, it appears that the man spent the first forty-five years of his life
preparing for his seven years of discovery: Born into a moderately well-to-do and
educated family, young Ampere had most of the advantages available to French children
reared during the Great Revolution. Moreover, he was a child prodigy who learned
geometry and calculus at the age of twelve by reading texts that were written in
their original Latin.
When Ampere was eighteen, his father was executed during the bloody "Reign of
Terror" that swept France. The sights and sounds of the revolution, topped off by
his father's violent death, shocked Ampere's mind. He spent the following six years
of his life wandering aimlessly about the countryside, building sand castles by
the sea and composing nonsense poetry.
At the end of that lost period of time, Ampere married and settled down to a
more conventional style of living. His brilliant mind had returned, but the family
money was gone. So, Ampere took his first job as a professor at the University of
Bourgen-Bresses. Barely three years passed before his wife died, shocking Ampere's
mind into a stupor for another year.
Napoleon had heard about the talents of this unfortunate young genius, and he
offered Ampere a teaching position at a school in Paris. Discouraged with life,
but anxious to return to his work, Ampere accepted the position and he remained
there for the rest of his professional life.
Ampere began contributing papers on a wide variety of subjects, including chemistry,
mathematics, molecular physics and biology. At the time, his special interest was
in the theory of games. These papers were important to other scientists, but they
were not the sort that fall into the category of special greatness.
A New Discovery
On September 11, 1820, Ampere happened to attend
a demonstration of Oersted's new discovery. The demonstration showed that a current
flowing through a straight piece of wire makes a compass needle turn to a position
at right angles to the conductor. Even while this demonstration was still in progress,
Ampere must have thought, "Since one conductor carrying an electrical current can
exert a force upon a compass needle, why can't two current-carrying conductors exert
forces upon one another?"
Excited by the notion that current-carrying wires produce exactly the same kind
of magnetic forces as loadstones and permanent magnets, Ampere immediately dropped
all his other work and began investigating this "artificial" source of magnetism.
In seven days, Ampere developed the fundamental theories of electrodynamics, designed
and built the experimental setups, performed the necessary experiments, and presented
his findings to the scientific world. No other major scientific discovery has ever
been conceived and tested in such a short period of time. Ampere was, indeed, fully
primed for this week of great discoveries.
Two highly significant ideas emerged from Ampere's mind and experiments that
week. For one thing, he developed what we now commonly call the "right-hand rule."
According to this rule, with the thumb of the right hand pointing in the direction
of conventional current flow (positive to negative) through a wire, the curled fingers
of that hand indicate the direction of the resulting magnetic field. Oersted had
already concluded that magnetic lines of force emerge at right angles from the conductor.
Ampere, however, perfected the notion by making it possible to predict the sense,
or polarity, of that field.
The other important idea in Ampere's first paper concerned the attraction and
repulsion of two parallel wires carrying an electrical current. Ampere showed that
currents flowing through the wires in the same direction made them attract one another,
while currents flowing in opposite directions made the wires repel.
Ampere's discoveries about the direction of magnetic fields around a conductor
and the forces acting upon a pair of current-carrying wires are just as important
today as they were 150 years ago. What is perhaps even more remarkable is the almost
unbelievable simplicity of the lab equipment he used. He managed to open a whole
new technology using nothing more than a few lengths of copper wire, a compass,
and a couple of Volta batteries.
During the seven years after his preliminary announcement, Ampere's papers became
increasingly spiked with complicated equations. His early studies of geometry and
calculus were paying off. Other researchers in Europe had picked up some good ideas
from Oersted's work, too; but most of these people lacked the high level of mathematical
sophistication and creative insight Ampere possessed.
Back to the Laboratory
His work soon reached a point where
he had to return to the laboratory to confirm his equations. This time he had to
obtain precise figures for the amounts of current flow and forces between the conductors.
Using what was then a revolutionary new measuring instrument, the galvanometer,
Ampere was able to measure the amount of current flowing through the wires. His
own original work with coils of wire and solenoids, by the way, was directly responsible
for the invention of the very galvanometer he used.
Since he also had to know the exact amount of force two conductors exerted upon
one another, Ampere devised a couple of specialized instruments. One of them was
an ordinary laboratory balance that had a solenoid attached to one side of the beam.
This solenoid fit inside a larger one fixed to the bottom of the balance. Current
flowing through the two solenoids made the smaller one move inside the larger. By
placing calibrated weights upon a weighing pan on the opposite end of the beam,
Ampere could determine the exact amount of force the two sets of conductors exerted
upon one another.
According to the famous scientist, James Clerk Maxwell, Ampere's fundamental
equations had "leaped full grown and fully armed from the brain of the Newton of
electricity." Ampere's equations were practically complete even before he set out
to demonstrate their validity in the laboratory. Making up equations before running
the experiments was contrary to the accepted scientific procedure of the time, but
one simple fact silenced all critics - the equations and laboratory experiments
always agreed. And to honor this "Newton of Electricity," the International Congress
of Electricians named the basic unit of current, the ampere, after him.
Ampere was a hard worker as well as a scientific genius. Even while he was concentrating
on the job of building the foundations of electrodynamics, he taught classes at
the university. Perhaps this was a mistake. Ampere was noted for stopping his lectures
in the middle of a sentence while his mind wandered off onto some new idea or equation.
He also had a habit of letting his work at the blackboard meander into some new
line of mathematical reasoning, leaving his students to puzzle over the jumble of
incomprehensible figures related to some new idea in electrodynamics.
Ampere was, indeed, a classic example of an absent-minded professor. There can
be no doubt, though, that he was one of the most successful absent-minded professors
of all time. Unlike the blackboards that carried his ideas off into oblivion, Ampere's
basic equations stand essentially unchanged to this day.
- See Full List -
Right-Hand Rule of Electricity
The right-hand rule is a simple mnemonic tool used to determine the direction
of the magnetic field created by an electric current. This rule is widely used in
electromagnetism and is especially useful for understanding the interaction between
electric currents and magnetic fields.
To use the right-hand rule, simply extend your right hand with your thumb, fingers,
and palm facing the direction of the current flow. Then, curl your fingers in the
direction of the magnetic field. Your thumb will then point in the direction of
the magnetic field.
This rule is based on the observation by Scottish physicist John Ambrose
Fleming that a current flowing in a wire creates
a magnetic field that circles around the wire in a clockwise direction when viewed
from the end of the wire. The right-hand rule is a convenient way to remember this
relationship and apply it to more complex situations involving multiple wires or
other types of electrical components.
For example, consider a simple loop of wire carrying a current. According to
the right-hand rule, the magnetic field created by the current will circulate around
the wire in a clockwise direction when viewed from the end of the wire. If we then
place a bar magnet near the wire, the magnetic field created by the current will
interact with the magnetic field of the bar magnet, producing a force on the wire.
The direction of this force can be determined using the right-hand rule.
Posted February 9, 2023
(updated from original post on 7/25/2017)