April 1960 Popular Electronics
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 very nice primer
on capacitors that appeared in the April 1960 issue of Popular Electronics.
A lot of ground is covered including history, form factors, dielectric types (ceramic
mentioned as a new variety at the time), applications, etc. Interestingly, units
of picofarads (pF) were still being referred to as μμfarads. In fact, since not
a lot of work was being done yet in the gigahertz (GHz) realm, there was not much
use for pF other than maybe to tune a filter response. Author Ken Gilmore reveals a sense
of humor when writing of early capacitance experiments as he says, "Since they couldn't
think of much to do with the Leyden jar except stand around and shock each other,
they didn't have any need for an accurate system of measuring the stored charge,
or the capacitance, of the jar."
The Capacitor: What It Is, What It Does, How It Works
By Ken Gilmore
The capacitor was invented in 1745 by experimenters who were looking for a way
to "condense" and store that newly discovered curiosity, electricity. Although many
of their ideas were wrong, they came very' close to doing what they set out to do!
Today's modern capacitor comes in thousands of different sizes, shapes, and colors.
It is of vital importance in the operation of everything from the family car to
guided missiles; yet it does exactly the same thing and works on the same principle
as its remote ancestor discovered in a laboratory at the University of Leyden over
two centuries ago.
Storing the Charge - What Is a Capacitor?
A great bolt of lightning crashes to earth with an ear-splitting clap of thunder.
This is perhaps the . most dramatic demonstration of capacitance at work.
A guided missile streaks into the heavens on a column of flame. Without capacitors
doing hun-dreds of different jobs in its guidance, control, and firing systems,
it would never leave the ground.
Your radio and television sets bristle with capacitors used in dozens of different
ways. Radio and TV broadcasting stations use thousands of them.
Neither your electric refrigerator nor your
car would start without capacitors; your fluorescent lights would remain dark.
Capacitors set off photographers' flash bulbs, help deliver electric power efficiently
to your home, automatically start water fountains and open doors as you approach
What is this strange phenomenon of capacitance that surrounds us on every hand?
How does it work? What causes it? What does it do?
The answer sounds almost too simple. A capacitor is a device that can store an
electrical charge. Because of this seemingly modest accomplishment, it can perform
an astonishing variety of jobs and is one of the most important of our electrical
and electronic servants.
Capacitive Operation - How a Capacitor Works
Did you ever walk across a carpet on a cool, dry day and feel a spark jump from
your fingers to the door knob as you reached to open the door? Whether you knew
it or not, your body was one part of a charged capacitor; the walls of the room
- including the door and the door knob - were the other part. You built up the electrical
charge by walking across the rug. The friction between your shoes and the rug deposited
excess electrons on your body, each one helping to build up a higher and higher
negative charge. Simultaneously, a positive charge of exactly the same strength
was accumulating on the walls.
When you got close to the door, the capacitor was discharged. The excess electrons
in your body leaped across space between your fingers and the door knob to neutralize
The capacitor formed by your body and the room is very different from the ones
used in radio, but it works in exactly the same way. A radio capacitor is usually
made of two or more metal plates, parallel to each other, but not touching. They
are charged, not by rubbing them across a carpet (it could be done that way, but
there is a better method), but by connecting them to a battery with a switch as
shown in the diagram to the right.
Charging the Capacitor
With the switch open, there is no charge across
the plates. When the switch is closed, the battery's positive terminal begins to
attract free electrons from the plate connected to it, while at the same time the
negative terminal starts to force large numbers of excess electrons into the plate
connected to it. More and more electrons pile up on the plate, making it continuously
more difficult for the battery to force any more on to it. Thus, one plate takes
on a negative charge, the other a positive charge.
Soon, the battery has moved all the electrons it can. The flow stops; the capacitor
is fully charged. If it were now disconnected and the voltage across it measured,
by a very high impedance meter, it would equal the battery voltage.
The capacitor actually stores the energy in its dielectric, that is, in the insulating
material between the metal plates. The dielectric can be air or any other insulator.
Practical capacitors are manufactured with dozens of different kinds of dielectrics,
This theoretical view shows how the charge is stored. In an uncharged capacitor,
the number of free electrons in either plate is the same. The electrons in the molecules
of the dielectric can be seen orbiting around their nuclei.
When a charge is applied, the picture changes. The negative plate now has all
of the free electrons. Since it is a basic law of electricity that like charges
repel each other, and unlike charges attract, the orbiting electrons in the dielectric
are repelled by the negative plate and attracted by the positive one. They move
as far toward the positive plate as they can, which stretches the molecules of the
dielectric out of shape. These misshapen molecules are like springs under tension:
they try to pop back to their normal shape.
As long as the charging voltage is applied, they can do nothing. But if a conducting
path is sup-plied between the two plates, the dielectric molecules will snap back,
pushing the excess electrons out of the negative plate, and discharging the capacitor.
The voltage storing ability of a capacitor is called capacitance. You may sometimes
hear it called capacity, but capacitance is grammatically correct.
Firing a Flash Bulb
Of what practical use is a capacitor's ability to store a charge? Photographers
use it in one of the simplest and most obvious ways. In one type of flash gun, they
charge a capacitor, then connect a flash bulb across its charged plates. All the
electrons stored on the negative side try to rush to the positive plate at one time,
through the flash bulb. This surge of current fires the bulb. (See the circuits
at the top of the next page.)
Why not connect the battery directly to the bulb? This could be done if a large
enough battery were used. Such a heavy-duty battery could deliver enough current
to fire the flash bulb. But a far lighter, more compact unit weighing only a few
ounces can be made to do the same job with the help of a capacitor.
A battery capable of putting out only a trickle of current - far less than would
be required to set off the bulb - can be used. Over a period of time, the trickle
builds up a powerful charge across the capacitor, in the same way that a tiny stream
of water can eventually fill a large tank. When the capacitor is fully charged,
it can deliver a surge of current even more powerful than the heavy battery, and
thus easily fire the flash bulb.
Positive and Negative
A great deal has been said about "positive" and "negative" charges. But
did you ever stop to think why one pole of a battery is called positive and the
other negative? It's all a mistake, really, because the one we call negative is
actually positive, and the one we call positive is... But maybe we should start
from the beginning.
Old Ben Franklin made the original mistake. Nobody knew for sure in which direction
current flowed. So Franklin guessed. He named one pole positive, the other negative,
based on the reasoning that current went from the positive pole, which he visualized
as having an excess of current, to the negative pole, which had a shortage.
He had a fifty-fifty chance of guessing right, but luck was against him. Many
years later it was established that current actually flows in the other direction.
By that time, positive and negative terminology was firmly established and it was
decided that no change would be made.
Whether the labels are right or wrong, polarity is an important consideration
in many capacitor circuits. For example, the electrolytic capacitors used in power
supplies will be ruined if they are connected with the wrong polarity.
Capacitors in Power Supplies
Capacitive "filters" are frequently used in power supplies to smooth out the
pulsating d.c, output from an a.c, rectifier circuit, thus allowing 117-volt house
current to be converted into direct current.
Without a capacitive filter, a power supply produces pulsating direct current.
The current Bows in only one direction, but not steadily. A picture of pulsating
d.c. from a full-wave power supply looks like this:
But radio and TV receivers need a source of pure d.c, that rises to a certain
voltage level and stays there.
A capacitor connected across the power supply gives just this effect. As the
voltage rises to maximum, the capacitor becomes charged. When the power supply voltage
falls to zero again, the capacitor begins to discharge, and helps keep the voltage
near its maximum level until the following power supply surge, which charges the
capacitor again for the next cycle.
You may notice that the voltage does not remain exactly at the maximum level
during the capacitor discharge. But if circuit components of the proper values are
selected, it stays close enough so that the difference is unimportant.
It is easy to tell when the filter capacitor (or capacitors) in your radio are
going bad. As the capacitor starts to fall down on the job, the ripple gets bigger
and bigger. Soon it begins to affect the operation of the whole set, and you hear
a loud hum. As it gets worse, speech and music become distorted or garbled; then
a heavy hum is about all you can hear.
Applications in A.C. Circuits
The two examples of capacitor use mentioned so far - photo flash and filter -
deal with d.c. voltages and currents. But a capacitor's function in a.c. circuits
is perhaps even more important. To understand how it works, let's take a look at
the two plates and battery setup again. Only, this time, they are connected - by
a double-pole, double-throw switch, that is, a switch that can quickly reverse the
polarity of the charging current applied to the capacitor.
With the switch thrown to the left, the capacitor charges. Open the switch, and
the capacitor retains its charge.
The switch is now thrown to the right. This connects the capacitor to the battery
again, but with the polarity reversed; the negative plate is now connected to the
positive battery terminal and vice versa.
The electrons quickly flow through the battery from the negative plate to the
positive one, discharging the capacitor. It then charges again, but this time with
opposite polarity. The ammeter shown connected in series with one plate will indicate
current flow during this process.
With the switch to the left, the meter will show a current flowing while the
capacitor is charging. When the switch is reversed, the meter indicates a current
in the opposite direction while the capacitor discharges its old charge and takes
on the new one. If the switch is thrown back and forth fast enough, the meter will
show current flowing at all times - first in one direction, then the other.
Thus, it is clear that even though direct current cannot flow in a capacitor
circuit (except during a brief charging period), alternating current can be made
to flow continuously by alternately charging and discharging the capacitor. To put
it another way, a capacitor "blocks" d.c., but "passes" alternating current. This
ability is put to work in countless ways. Here, for example, is a simplified amplifier
circuit that demonstrates the effect.
The signal is introduced into the tube's grid circuit, is amplified, and leaves
through the plate circuit. For the tube to work, the plate must be kept at a high
positive voltage-say 200 volts-while the grid must be slightly negative.
Since electron tubes usually operate with a high positive voltage on the plates
and a low negative voltage on the grid, the problem obviously arises: how can the
tubes be coupled together plate-to-grid without disturbing their respective d,c,
The capacitor is made to order for this job. Since the signal to be amplified
is a.c., it will pass through a capacitor easily, while the d.c, operating voltage
will be blocked.
A capacitor used in this way is called a coupling or blocking capacitor. Either
name is correct.
A capacitor's ability to pass a.c. while blocking d.c, is also useful in another
kind of hookup. For example, signals frequently appear where they aren't wanted.
A capacitor can "short" such an unwanted signal to ground while leaving the circuit's
d.c, voltage unaffected. This is called "by-passing."
Versatility Unlimited - Values, Types, Uses of Capacitors
The capacitor was invented back in October, 1745, by Dean E. G. von Kleist of
the Kammin Cathedral in Pomerania. A few months later - in January, 1746 - Pieter
von Musschenbroek, a professor at the University of Leyden, made the same discovery
all over again. Somehow, Musschenbroek got the credit, and early capacitors were
called Leyden jars after his university. You may have seen one around a physics
laboratory; they're still used at times to demonstrate the principle of capacitance.
The Leyden jar is simply a bottle with about three-quarters of both the inside
and outside surfaces covered with metal foil. The two pieces of foil are insulated
from each other by the glass dielectric. A brass rod goes through a stopper and
makes contact with the inside foil.
Early experimenters used a jar because they were looking for a way to "condense"
and store electricity. Since they thought of electricity as a fluid, they figured
a jar would be just the thing to hold it. The name condenser, which is still frequently
used instead of capacitor, comes from these early attempts to condense electricity.
Musschenbroek and his associates discovered that if they touched the brass rod
of the Leyden jar to an "electric machine" (they had a crude electrostatic generator),
the jar retained a charge. You could get a shock by holding the outer foil with
one hand and touching the rod with the other.
Since they couldn't think of much to do with the Leyden jar except stand around
and shock each other, they didn't have any need for an accurate system of measuring
the stored charge, or the capacitance, of the jar.
As the science of electricity progressed, it became obvious that a system of
measurement was needed. So a basic unit of capacitance was decided on. It was named
the farad, after Michael Faraday, one of the great electrical pioneers.
A farad represents a specific amount of "storing power" or capacitance. In actual
use, the farad turned out to be far too large a unit, so practical capacitors are
usually rated in microfarads (mf.) - one millionth of a farad, and in micromicrofarads
(mmf.) - one millionth of a microfarad. (According to one system of notation, a
"μ" is substituted for "m" in the abbreviation. Thus, "mf." becomes "μf." and "mmf."
becomes "μμf." The meaning in either case is the same.) To put it another way:
1 mf. (or μf.) = .000001 farads
1 mmf. (or μμf.) = .000000000001 farads
The capacitance of any capacitor is determined by four factors. Let's take a
look at each one.
1. Size of plates. Large plates can hold a greater charge (more electrons)
than small plates.
2. Separation of plates. The closer together the plates are (without
touching), the greater the charge they can store.
3. Number of plates. The more plates, the greater the capacitance.
4. Dielectric constant. Every different dielectric material has its
own dielectric constant. Air has an arbitrarily assigned constant of 1. Mica has
a constant of about 7. This means that mica will store about seven times the charge
that air can handle, with all other factors the same. Paper has a dielectric constant
of about 5, and some types of ceramic over 1000! Different substances have different
constants because each molecule has a different "natural elasticity," which allows
some to store vastly greater amounts of energy than others. Among frequently used
dielectrics are plastics of various kinds, air, mica, and paper.
Types of Capacitors
Here are some of the more common types of capacitors, classified according to
Paper capacitors are made of long strips of aluminum foil, wrapped tightly in
a roll, separated by a paper dielectric. To make the paper a better insulator (to
prevent breakdown of the capacitor when high voltage is applied to its plates),
it is usually impregnated with oil, wax, or plastic.
Plastic capacitors are similar, but use thin sheets of plastic - Mylar
and others - as a dielectric. They have the same uses and are about - the same size
as paper capacitors.
Metallized paper capacitors are another variation of the same basic
type. Instead of strips of aluminum foil, this capacitor's plates are microscopically
thin layers of metal deposited by an evaporation technique on the dielectric paper.
Because the plates are so thin, the capacitor can be rolled into a very much smaller
package than a standard capacitor of the same capacitance.
All of these variations of the paper capacitor are widely used in coupling, bypass,
and tone control circuits. They are usually tubular, and range in capacitance from
about 250 μμf. to 1.0 μf. or more. They have voltage ratings up to about 1600 volts
- that is, they can withstand 1600 volts without the voltage breaking through the
dielectric and destroying the capacitor. Most used, however, are capacitors in the
400-600 volt range.
Minor differences exist between the various types. Plastic capacitors can be
built more easily to withstand higher voltages. Metallized ones, as mentioned earlier,
are smaller, and they cost more. With these exceptions, the three types are generally
The printed band around one end of these tubular capacitors tells you which lead
is connected to the outer layer of foil. In general, the lead so marked should be
connected to the "low" side of the circuit. To put it another way, connect it to
ground if possible, or to the side of the circuit electrically closest to ground
potential. The band does not indicate the polarity of the connections. When a capacitor
is used this way, the outer layer of foil serves as an electrostatic shield, so
that the capacitor's operation will not be affected by other stray fields within
Oil capacitors also use a layer of paper as a dielectric; the paper
is impregnated with a special type of oil that gives it both a high capacitance
and a high voltage rating. They are usually used as high-voltage power supply filters.
Capacitance varies from 1.0 μf. to 20.0 μf. or more.
Oil capacitors are usually sealed in a heavy can, and may have a rating of 1000
volts or more.
Mica capacitors are made of a number of flat strips of metal (tin, copper, aluminum,
etc.) separated by sheets of mica. Alternate plates are hooked together, and the
whole assembly is molded into a block of plastic or ceramic material.
They range in capacitance from about 10 μμf. to .01 μf. Mica is an unusually
good insulator, so capacitors with a mica dielectric can be built with ratings up
to 5000 volts or more, and are used in high-voltage transmitting circuits.
Ceramic capacitors, a newer type, use sheets of ceramic as a dielectric. The
plates are normally vapor-deposited silver. A ceramic capacitor generally has only
two plates - one on either side of a ceramic disc, or one on the outer and one on
the inner surface of a ceramic tube.
Since ceramic has a very high dielectric constant, up to 1200, relatively large
values of capacitance can be obtained with small capacitors. Also, the insulation
quality of ceramic is excellent, so these units can easily be designed to operate
at several thousand volts. They are widely used in television, military and satellite
communications equipment, and other critical circuits.
Advanced manufacturing techniques have brought the cost of ceramics down to about
the same range as paper. They have one disadvantage: they are not as readily available
in the larger common values.
Electrolytic capacitors pack the largest amount of capacitance into
the smallest space. They come in sizes up to several thousand microfarads, with
working voltages up to about 600 volts. The cans an inch or more in diameter and
four to six inches long that are mounted on top of almost every radio and TV chassis
are electrolytic capacitors. They are usually used as power supply filters.
Electrolytics have extremely high capacitance values because the dielectric is
only a few millionths of an inch thick. The capacitor is manufactured by dipping
an aluminum sheet into an electrolytic solution, and setting up a current flow from
the solution to the aluminum. The action of the current builds up a layer of oxide
on the plate. When the layer is completely formed, the aluminum is ready to become
the positive plate of a capacitor. The dielectric - the oxide coating - is already
in place. The unit is sealed in a can filled with a conducting liquid which becomes
the negative plate of the completed capacitor.
This is a description of the so-called "wet" electrolytic. There is also a "dry"
electrolytic. The only difference is that the "wet" uses an actual solution, while
the "dry" has a saturated layer of gauze between the plates. In actual use, the
wets have almost disappeared, because drys are more convenient to manufacture, store,
Electrolytics, like most other components, are getting smaller and smaller in
this age of miniaturization. A recently developed type - the etched aluminum electrolyte
- packs even greater capacitance into a smaller volume by using a plate that has
been roughened by chemical etching. A greatly magnified cross section of the etched
aluminum would compare with the usual polished surface like this:
Obviously, the etched plate has a far greater surface area exposed to the electrolyte,
and consequently has greater capacitance. The etched aluminum capacitor is now on
the market, but is considerably more expensive than the ordinary electrolytic. Its
extra cost is, of course, well worth the difference in such diverse applications
as hearing aids and missiles, where weight and size are very important.
Electrolytics have several disadvantages. One is that leakage current is larger
than for any other type. The other is that the electrolytic has a positive and a
negative terminal. Therefore it cannot be used where the polarity changes (in a.c,
circuits, for example). Great care must be taken to see that it is properly connected.
Even a few seconds ex-posure to the wrong polarity voltage can ruin an electrolytic,
or even cause it to explode.
Variable air capacitors are used in every radio for tuning in different stations.
One of the sets of plates is fixed to the frame, and is called the stator. The
other set, which moves, is called the rotor. Naturally, as in all capacitors. the
two sets of plates are close together but do not touch. Capacitance is varied by
chang-ing the amount the plates mesh. (There are fixed air capacitors. but they
Variable air capacitors come in sizes from a fraction of a μμf. to 1200 μμf.
or more. Those used in low-voltage receiving circuits may have 10 to 30 plates separated
by less than a hundredth of an inch. Large transmitting types can have 80 to 100
plates. separated by a half inch or more.
Variable air capacitors are frequently ganged. This means that several independent
capacitors are arranged along one shaft so that they rotate together. In this way,
several circuits can be tuned simultaneously.
Although we have only mentioned fixed mica, paper, oil, ceramic, and plastic
capacitors, there are variable capacitors which use some of these dielectrics, too.
But most variable capacitors have air as the dielectric. The one common exception
to this is the small mica "trimmer" capacitor found in most radio receivers. These
units, with a capacitance of only a few μμf., are adjustable with a screwdriver.
They are used in making minor adjustments in circuits where the amount of capacitance
is critical. The local oscillator in a superheterodyne receiver, for example, is
tuned to an exact frequency with a mica trimmer.
Only the principal types of capacitors have been listed so far. There are many
others: vacuum, glass, vitreous-enamel, polystyrene, tantalum, Milinex, and even
one with the jaw-breaking name of polytetrafluorethlene. Each has its own advantage
and special uses. And some, like tantalum, are becoming more popular.
The many ways mentioned so far in which capacitors are, used hardly scratches
the surface of the jobs to which this versatile component is suited. Every radio,
television, or communications transmitter or receiver, for example, must operate
on a certain predetermined frequency. The signal sent out by the transmitter must
oscillate, or vibrate, at a precise rate - so many times a second. Receivers must
be tuned to this exact frequency to pick up the signal. Capacitors play an important
part in circuits which determine operating frequency. Vary the capacitance, and
the frequency changes. When you tune your radio, you are adjusting the capacitance
of the tuning circuits.
Another important duty of the capacitor is wave-shaping. The most common waveshape
is the sine wave.
The electrical power that comes into our homes is in this form; this is also
the shape of an ordinary oscillator's output. But for certain uses - radar, television,
telemetering, to name only a few - waveforms of many shapes must be produced.
These and thousands of other waveshapes can be formed by hooking capacitors together
in different combinations with other components.
Oh, yes, one more thing. What does capacitance have to do with lightning? In
stormy weather, air currents rise swiftly. Particles of water vapor in clouds are
swept past other stationary particles, and a charge accumulates by friction, just
as it does when shoes rub across a carpet. The charge on the clouds, small at first,
builds up rapidly. At the same time, a similar - but opposite - charge builds up
on the ground under the cloud. As the cloud races across the sky, the charge moves
along the ground with it - it can be measured with the proper equipment.
Higher and higher builds the charge, as more particles of water vapor rush by,
each adding to the charge. First, it can be measured in volts, then millions, next
trillions of volts from cloud to earth. Finally, the giant capacitor - the cloud
forming one plate, the earth the other - "breaks down." The charge arcs over the
insulating dielectric (air) and a blinding flash lights up the heavens. The mammoth
capacitor discharges in a brilliant flash of lightning.
Capacitance - the simple ability of two bodies to store an electrical charge
- is thus responsible for one of our most useful electrical components, and at the
same time, for one of nature's most spectacular displays.
Posted February 3, 2020(original 7/5/2012)