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.
Anyone who has lived in an area where the
humidity gets so low that getting an electrostatic discharge jolt when touching
a doorknob or or other isolated metallic object, will appreciate Mac McGregor's
discussion with Barney. Appropriately, it appeared in a February issue of Popular
Electronics magazine, the setting being Mac's Service Shop's Midwestern location
- a prime environment for receiving the annoying zaps. It is a timeless subject
for anyone routinely subject to exposed high voltages of any sort - some being more
dangerous than others. Most RF Cafe visitors already know that technically, it is
the amount of electric current through the body that determines severity of electric
shock, not the voltage. However, we also know that voltage does play a role because
a certain voltage, per Ohm's law, is needed to induce a commensurate current. The
body's resistance is determined primarily by perspiration (salt and water) and the
path between contact points (e.g., across adjacent skin areas or hand-to-hand via
the heart). MIL-STD-883 and JEDEC have decided that the proper Human Body Model
(HBM) for testing semiconductor survivability when subject to high voltages is 1.5 kΩ.
I could not find out how that value was determined.
The sparkling cold winter morning lifted the heart but numbed the fingers as
Barney sprinted quickly over the squeaking snow from his car to the service shop.
There he found Mac, his employer, seated at a service bench bearing some strangely
assorted paraphernalia: a red-handled tooth brush, a ten-inch-long glass rod, one
square of rough brown woolen cloth and another of pink silk, a couple of coat hanger
wire stands shaped like bridge lamps and carrying-pea-sized little white balls suspended
by silken threads from the ends of their horizontal arms, and a gaily decorated
round tin candy container.
"Okay, I give up," Barney said after a puzzled examination of these objects.
"What the heck are you doing?"
"In the parlance of the day, I'm trying to 'get it all together,' " Mac answered
with a teasing grin. "I'm going back to where our line of work really started when
Thales of Miletus, about 600 BC, observed that a piece of rubbed amber, called "elektron"
in Greek, attracted bits of matter. All the millions of uses for electricity and
electronics in our modern civilization can be traced back to that casual observation
of electrostatic, or triboelectric, charge. Deciding a review of basic electrostatic
principles would not hurt me, I got some books from the library, made those little
balls from pith gouged out of the center of branches lopped off trees of paradise,
or stink trees, growing in my back yard, made the simple leaf electroscope contained
in that candy tin with chewing gum wrapper foil, invoked the spirit of Ben Franklin,
and started experimenting, trying to explain everything I saw happen in terms of
what I know about electron theory. Never before did I get so much thought-provoking
pleasure from such simple home-made apparatus."
The leaf electroscope is made from a 5" in diameter round candy
tin with the bottom cut out. A small porcelain feed-through insulator goes through
a hole in one side of the can, and a brass tube with internal threading is screwed
onto the bottom end of the insulator. The bottom end of this small-diameter tube
is split, and two leaves of foil 1/4" by 2" have their ends clamped in the split.
I used thin metal foil from chewing gum, and beat it even thinner. Gold leaf, obtainable
from a sign painter, would have been better. Plastic wrap is stretched over both
ends of the can to allow the leaves to be seen while protecting them from air currents.
A brass ball could well replace the metal washer on top of the insulator. When a
charge is placed on the washer, either by contact with a charged object or by induction,
like charges on the leaves cause them to spread apart. They then collapse when the
charge is subsequently taken away.
Electrostatic Induction with Leaf Electroscope
"Aw, I did all that stuff in high school physics," Barney scoffed. "Electrostatic
experiments are interesting but of little practical value except to explain how
lightning rods work."
"How easy it is to be so cocksure - and so wrong - when you are young!" Mac marveled.
"Did you learn that, on a clear bright winter day such as this, the downward electrostatic
charge in the atmosphere may carry up to 500 volts per vertical meter?"
"Don't believe it," Barney answered promptly. "That would mean there would be
almost 1,000 volts from my head to feet. That would electrocute me."
"Not so. You constitute a grounded conductor, and your skin is an equipotential
surface that warps the electric field and makes you unaware of it, even when a thundercloud
moves overhead and reverses the field polarity and increases the potential up to
10,000 volts/meter."
"That's when the lightning strikes," Barney interrupted.
"It's not that simple. You need 300 times that voltage, or 30,000 volts/cm, to
break down the resistance of air. Actually a 'leader' stroke develops stepwise inside
the cloud and comes to ground; then there is a main upstroke along the ionized path
of the leader containing tens of thousands of amperes. By the way, did they tell
you about earthquake-lightning in your physics class?"
"Nope. We didn't believe in compounding catastrophes."
"Nature apparently does. Flashes of light in the sky often accompany earthquakes.
During the Japanese quake of 1930 some 1,500 such flashes were recorded. That quake
area is characterized by quartz-rich lava, and it has been suggested that, with
the right kind of crystalline order and the right kind of seismic waves, millions
of volts of electrostatic energy might be generated by the earth's movement of the
rock formation through the piezoelectric effect - the same effect that produces
the weak voltage across the output of a crystal phono cartridge when the stylus
vibrates in the record groove. Perhaps if any quartz-bearing areas can be found
along the San Andreas fault, stations for continuous monitoring of the atmospheric
electric field can be set up and their recordings correlated with ground tremors.
If these coincide, this might lead to an earthquake early warning system."
"You still haven't shown me that electrostatic electricity is practical."
Practical Applications. Before answering, Mac rubbed the toothbrush handle with
the woolen cloth and then held the handle near one of the pith balls. The ball was
attracted to the handle and clung to it for a few seconds and then leaped violently
away and swung over to a metal meter panel and clung to it.
"That should suggest one very practical use: a precipitator for removing air-polluting
fly ash and other liquid and solid particles from flue gases," he said. "All we
need do is put an electrostatic charge on the particles, such as I put on the pith
ball, and subject them to a field so they will move toward and cling to an oppositely
charged or neutral surface. In practice, this can be done by running a thin wire,
carrying a negative potential of 100,000 volts, down the center of a cylindrical
duct 20 cm in diameter. The charge produces an average radial field strength of
10,000 volts/cm, but the field strength is much less near the duct wall and much
more near the wire. In fact, in the immediate vicinity of the wire it is far above
the 30,000 volts/cm I mentioned as being necessary for breaking down the resistance
of air. This results in a corona discharge, or zone of ionization, around the wire.
Electrons surging off the wire attach themselves to oxygen molecules of the air,
converting them into negative ions that are repelled by the wire so they move outward
toward the grounded duct wall in a veritable ionic current.
"If a flue gas loaded with waste particles flows up the duct with a velocity
of less than ten feet a second, the ionic current charges the particles and makes
them move across the gas stream by the billions to collect on the walls of the duct.
If the particles are dry, the duct is rapped so the ash falls downward and is collected
in a hopper. Liquid particles simply run down the duct walls. Industrial precipitators
operate on a negative corona, while home air cleaners use a positive corona. It's
estimated such devices trap more than twenty million tons of fly ash a year. I'd
call that a practical use."
"So maybe there is one practical use," Barney admitted.
"There's much more. The principle of corona discharge is also used to separate
granular mixtures in which the two kinds of particles differ in conductivity so
one might be called a conductor and the other an insulator. Remember conductivity
is always a relative term. In one form, the mixture comes down from a hopper and
spreads out in a thin layer on top of a grounded rotating drum. The drum passes
under a wire generating a corona discharge. Ions flood through the mixture to the
drum. They pass through the conducting particles to the drum and there is no adhesion;
so these particles simply fall off into bin #1 as the drum turns. The charges of
ions that strike the insulating particles coat the particle surfaces with a charge
that pins them to the metal drum while it moves past bin #1, and they are scraped
off in bin #2. This kind of separator is used extensively to separate iron ore,
but it is also used to remove rodent excreta from rice, extract garlic seeds from
wheat, and to separate nut meats from shells.
"In the handling of continuously moving sheets of paper or film, one surface
of which is coated with a sticky substance, the 'web' can be pinned to the surface
of a single roller to supply tension by charging the outer surface with ions supplied
by a corona discharge.
"Still another important use of the corona discharge is electrocoating, a process
used to apply various coatings such as wet paint, grit particles, dry powders, and
even short fibers. A spray gun equipped with a corona point emits a fine mist of
paint particles that gather the field lines to themselves and attract ions from
the corona, thus acquiring a charge. The charged particles are so strongly attracted
to the grounded target that they actually curl around it and coat the sides and
back surface. It's estimated the saving in paint alone from electrocoating amounts
to $50 million a year.
"Flocking is a variation of electrocoating. If you want a velvet wall, you first
paint it with conductive aluminum paint to which an adhesive is applied. Then you
fill a hopper with short fibers and shake it in front of the wall. As the fibers
fall out they are charged from a set of corona points mounted on the hopper, and
three things happen: (1) the fibers are driven toward the wall by the Coulomb force
of repulsion of like charges, ( 2) the mutual repulsion of like charges on the fibers
keeps them apart, and (3) the fibers align themselves with the lines of force so
they arrive end-on at the adhesive, permitting more than 200,000 fibers per square
inch to be applied. This process is used to make artificial suede, cover the interior
of instrument cases, or put pile on carpeting. A similar process is used in the
$200 million a year business of coated abrasives, such as sandpaper and emery paper."
"Okay! I'm convinced. Electrostatic electricity is more than a toy," Barney conceded.
"There's more," Mac said relentlessly. "Let's talk about the dry-copy imaging
process known as xerography. The operation of a Xerox machine depends on the fact
that a selenium-covered plate can be charged by a corona discharge, and then the
charge can be removed by exposure to light. In actual operation a selenium-coated
drum is charged in the dark from a corona, and then an image of the page to be copied
is focused on the drum. The charge is removed in the light areas but retained in
the dark areas. Next a 'toner,' a mixture of black dust and tiny glass spheres,
is spread over the image. The opposite-charged glass and dust stick together until
the mixture reaches the image; then the glass is repelled and the dust clings to
the dark areas.
"Now paper that has been charged is spread over the image on the drum and attracts
the toner to itself. Finally this paper moves through a rapid-heating stage that
fuses the toner to itself and makes a permanent copy. This is a simplified explanation,
of course, but I'm sure that you will get the idea."
"By the way, where did you learn all this stuff anyway?"
"From various books and magazine articles. One of the best sources was the work
of A.D. Moore, professor emeritus of electrical engineering at the University of
Michigan. Two of his books are Electrostatics and Invention, Discovery and Creativity.
He was working on another that may be published by this time called Electrostatics
and its Applications. In an article in the March, 1972, issue of Scientific American
he points out that Ben Franklin invented the first electric motor, an electrostatic
motor; and he goes on to say interest in this type of motor has been revived recently,
chiefly by Oleg Jefimenko of West Virginia University. One of his corona
motors about five inches long developed a tenth of a horsepower. Recently he put
up a wire by balloon and ran one of his motors by energy from the atmosphere's electric
field."
"That does it!" Barney exclaimed. "I'm going home tonight and dig out my physics
books. How about borrowing those playthings - excuse me, that apparatus - of yours?
"Con mucho gusto," Mac replied, grinning. "That was the whole idea. You'll have
fun, and, as a bonus, I'll guarantee it will be much easier to understand solid-state
electronics after you've reviewed your electrostatic electricity."
Posted July 12, 2024 (updated from original post
on 5/29/2016)
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive
technodrama™
stories was the brainchild of none other than John T. Frye, creator of the
Carl and Jerry series that ran in
Popular Electronics for many years. "Mac's Radio Service Shop" began life
in April 1948 in Radio News
magazine (which later became Radio & Television News, then Electronics
World), and changed its name to simply "Mac's Service Shop" until the final
episode was published in a 1977
Popular Electronics magazine. "Mac" is electronics repair shop owner Mac
McGregor, and Barney Jameson his his eager, if not somewhat naive, technician assistant.
"Lessons" are taught in story format with dialogs between Mac and Barney.
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed
formulas and reference material while performing my work as an RF system and circuit
design engineer. The World Wide Web (Internet) was largely an unknown entity at
the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps
while tying up your telephone line, and a lady's voice announced "You've Got Mail"
when a new message arrived...
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All trademarks, copyrights, patents, and other rights of ownership to images
and text used on the RF Cafe website are hereby acknowledged.
