December 1972 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Popular Electronics,
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. As most RF Cafe visitors know, both a steady state and timevarying current will
generate a magnetic field, but only a timevarying magnetic field can generate a current
flow. In less than a week after witnessing
Hans
Christian Ørsted's demonstration of a currentcarrying wire influencing a compass
needle, Ampere discovered the
RightHand Rule of current flow direction based on the direction of
the magnetic field.
Ampere's Law
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 electrodynamics  and he did it in just seven years.
Early Years. Looking back at Ampere's work from our presentday point of view, it appears
that the man spent the first fortyfive years of his life preparing for his seven years of
discovery: Born into a moderately welltodo 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 BourgenBresses. 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 currentcarrying
conductors exert forces upon one another?"
Excited by the notion that currentcarrying 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 "righthand 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 currentcarrying 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 absentminded professor. There can be no doubt,
though, that he was one of the most successful absentminded professors of all time. Unlike
the blackboards that carried his ideas off into oblivion, Ampere's basic equations stand essentially
unchanged to this day.
Posted July 25, 2017
