August 1973 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.
Biographies focusing individually
on Albert Einstein, Thomas Edison, Isaac Newton, Alexander Graham Bell, and Henry Ford abound.
Someone, somewhere, is right now researching and writing yet another dissertation on each
them and other well-known historical figures of science and engineering. Guys like
rarely have books dedicated solely to their lives and accomplishments, even though it is not
unreasonable to expect that they would. Faraday,
Max Planck, et al,
are usually included in books featuring a collection of people who have achieved notoriety
in similar fields. Accordingly, most of us know little, if anything, about their upbringings
or what led to their claims to fame. Here is a brief insight into just those aspects of the
man whose namesake is the root of units of capacitance and identifies a common type of shield
(aka "cage") used to isolate devices from external electromagnetic fields.
In case you are interested: "Michael Faraday: Father of Electronics," "Volta: Science and Culture in the Age of Enlightenment," "Georg Simon Ohm," "
Mémoires sur l'Électromagnétisme et l'Électrodynamique" "Anders Celsius (Scientists at Work)," "Planck: Driven by Vision, Broken by War"
Faraday and Electrostatic Lines of Force
By David L. Heiserman
As a boy in the early 1800's, Michael Faraday hardly looked
the part of someone destined to grow up to become one of the world's most productive scientific
geniuses. The son of a poor London blacksmith, Faraday spent much of his boyhood standing
in welfare lines waiting for food. Out of sheer desperation, his family permitted him to drop
out of school at age 13 to earn his own way as an errand boy in a bookstore. For young Michael,
leaving school was no great loss since he had no fondness for school.
Faraday soon found that he had a liking for books, especially the ones about popularized
science. Fortunately, his employer was an understanding man who allowed the boy to read between
errands and janitorial duties.
One day a customer gave Faraday a ticket to a lecture that was to be given by the eminent
British scientist, Sir
Humphry Davy. Owing to his reading, Faraday understood most of what Davy said at the lecture.
He also managed to take down an incredibly complete and accurate set of lecture notes. A few
days later, he copied the notes into a booklet and mailed them to Davy, along with a request
for any kind of a job in the scientist's laboratory. Davy was impressed and flattered and
offered Faraday a job as bottle washer in his chemistry lab.
The Scientist Emerges. Faraday's abilities and enthusiasm soon prompted
Davy to promote him to research assistant. After that, Faraday's list of accomplishments makes
most success stories seem uneventful. By the time he was 30, he had worked his way up to being
one of Europe's most popular experimenters and lecturers. Almost entirely self-taught, Faraday
conducted his own experiments in chemistry and electricity with a genius and precision that
surpassed that of most scientists of his time. He had no liking for mathematics, but he made
up for the deficiency by drawing elaborate analogies between everyday situations and his abstract
The Coulomb Torsion Balance. The instrument is initially calibrated by
turning the torsion micrometer so that it reads zero when the two pith balls just touch. Placing
an electrical charge onto the pith balls makes the suspended ball rotate away from the fixed
one. Faraday rotated the torsion micrometer until the charged pith balls were exactly 20°
apart, then he recorded the number of degrees and minutes he turned the micrometer knob. By
knowing the torsion constant of the glass thread, it was possible to calculate the amount
of force and, hence, the amount of the electrical charge that was on the balls.
In 1831, Faraday began his famous series of experiments that eventually led to his discovery
of electromagnetic induction and the invention of the first electric motors and generators.
It was only his lack of mathematical sophistication that prevented Faraday from becoming the
discoverer of radio. Clerk Maxwell, a more mathematically minded investigator, later used
Faraday's principles to formulate the basic equations for electromagnetic waves.
Ever in search of new knowledge, Faraday by 1836 returned to the electrolysis experiments
he had once shared with Davy. By placing sheets of metallic foil on opposite faces of a block
of ice, he demonstrating that no current could flow through the ice until it melted. To the
contrary; the ice seemed to gather and store an electrical charge. But once the ice melted,
current began to flow and decompose the water into its fundamental elements of oxygen and
hydrogen. While this experiment was popular among professional and amateur experimenters of
the time, Faraday saw some important features in it that others had overlooked.
The idea that current-carrying conductors produce a magnetic field had served him quite
well in his work with induction; so, he proposed the existence of another kind of field -
an electric field - to explain the storage quality of ice and all other kinds of nonconductors.
In his laboratory notes dated December 23, 1836, Faraday describes a new kind of apparatus
for studying the relationships between different types of insulating materials and their "inductive
Faraday built devices made of two hollow airtight brass spheres. One of the spheres was
small enough to fit inside the other, leaving 1/2 inch of space all around for inserting different
types of insulating gases or solids. He suspended the smaller sphere inside the larger by
means of a glass tube. A wire running through the glass tube provided electrical connection
to the inner sphere.
The outer sphere was mounted on a stand equipped with a valve that let him evacuate the
space between the spheres or fill the space with different kinds of gases. He also fashioned
a mold for forming solid materials that would perfectly fit into the space.
With this apparatus, Faraday was able to construct a "spherical" capacitor whose plates
he could separate with any type of dielectric material of his choosing. His main idea was
to compare the "inductive capacities" (a term now known as "dielectric constant") of different
insulating materials by charging the spheres with a static potential and measuring the amount
of charge they acquired.
To measure the stored charges, Faraday used a sensitive torsion balance invented by Coulomb.
This apparatus consisted of a thin lacquered straw about the size of a toothpick suspended
at right angles from a length of fine glass thread. Minute forces applied in the proper direction
to one end of the straw made the straw twist about the thread. By measuring the angle of the
twist, an experimenter could calculate the actual amount of applied force.
To make the torsion balance sensitive to electrostatic charges, Faraday attached a small
pith ball to one end of the straw. He attached a piece of paper to the opposite end to serve
as a damper for the mechanical oscillations and act as a counterweight. Another pith ball,
fixed to the frame of the balance, carried test charges to the space around the ball on the
straw. Charging the fixed pith ball made the movable one rotate through an arc Faraday measured
by means of a piece of paper scribed with units of arc in degrees and minutes.
A Faraday Sphere. Faraday used this piece of equipment to determine the
"specific inductive capacity" (which we now call the dielectric constant) of different types
of insulating materials.
In his experiments, Faraday would place a dielectric material between the spheres, charge
them with a static voltage, and measure the amount of charge with the torsion balance. He
also kept track of how fast the charges leaked off the spheres, discovering that different
materials took on different amounts of charge. Spheres separated by glass, for example, took
on larger charges and held them longer than did spheres separated by air or hydrogen. This
confirmed his suspicion that different insulating materials have different "specific inductive
What is more important, these experiments backed up his theory of electrostatic lines of
force and cleared up a longstanding problem concerning charged insulators. Other researchers
believed that the metallic plates and not the dielectric between them held the stored charges.
By demonstrating that electrostatic lines of force within the dielectric - not the plates
- held the stored charge, Faraday cleared up a prevalent misconception.
The Perfectionist. Since Faraday did not like to bother with mathematics,
he was content to explain his findings in terms of pictures which showed lines of force more
concentrated in some materials than in others. His data was so accurate and complete that
other scientists incorporated his notion of "specific inductive capacity" into formal equations
that stand to this day.
Faraday's notes indicate that he often mistrusted the readings he obtained from the torsion
balance. To convince himself, and many critics as well, that the amount of charge stored within
different types of materials was really different, he frequently used two identical sets of
hollow spheres. He would charge one set, measure the charge with the torsion balance, and
then touch the charged set of spheres to an uncharged set. Whenever the two sets contained
the same type of dielectric, they divided the original charge equally. But when one set of
spheres contained a better dielectric than the other, the set with the better dielectric took
on a large percentage of the original charge.
The scientific community accepted Faraday's theories and experimental results with enthusiasm.
As a result of his work with dielectric lines of force and dielectric materials, the 1891
International Electrical Congress voted to name the electrical unit of capacitance, the "Farad,"
in honor of Michael Faraday.
Posted August 29, 2017