Drawing of a hand holding a wire on a home made jar capacitor.


The subject of this chapter has two names-CONDENSER and CAPACITOR-and both names are OK. A lot of hot arguments go on over which is the correct name. Some men argue, and probably rightly so, that the name CONDENSER is wrong, since NOTHING is CONDENSED. These men prefer the term CAPACITOR, since the gadget has the CAPACITY to hold a charge. But actually, a charged condenser does not contain ONE MORE ELECTRON than when discharged.

What it boils down to is this-take your pick. You can use either term you choose. However, this book uses CONDENSER, because this term is older and probably more widely used.

Many natural condensers are formed. A cloud moving above the surface of the earth becomes charged, and possesses characteristics of a condenser. A bolt of lightning is simply the DISCHARGE of one of these natural condensers.

Your body, when you are near an improperly shielded or poorly constructed radio receiver, can also form a


condenser. You certainly have seen receivers with which it was necessary to hold your hand in one spot in order to get the program. With such receivers, your hand is as much a part of the circuit as a vacuum tube or a resistor.

Watch your step when handling condensers. They are interesting little gadgets that can do you wrong if you

Simple condenser.
Figure 87.-Simple condenser.
get hold of a "hot" one. Small condensers are not too dangerous, but big ones are capable of killing a person.


In physical structure the condenser is one of the simplest electrical devices used in radio circuits. Basically it is TWO METAL PLATES separated by an INSULATOR. Notice, in figure 87, the insulation is called the DIELECTRIC. It may be MICA, GLASS, WAXED PAPER, CERTAIN CHEMICALS IN SOLUTION, or AIR.

The physical appearances of condensers are as varied as 5th Avenue hats on Easter morning. Some are small and simple in design, others are large and complex.

The MICA CONDENSERS, figure 88, are the smallest. They have two metal plates separated by a sheet of mica. The whole unit is enclosed in a plastic case. Many of these mica condensers are about the size of postage stamps.

The TUBULAR CONDENSER, figure 89, is next in size to the mica condenser. Most are from 1½ to 2½ inches


Mica condenser.
Figure 88-Mica condenser.
long and 3/8 to 3/4 inch in diameter. They are formed by making rolls of two sheets of tin foil separated by waxed paper. Because of the paper dielectric, these condensers are often called PAPER CONDENSERS.

The edges of the individual sheets of tin foil are soldered together to provide the dove-tailed appearance of the illustration. If the two pieces of foil touch, the condenser is shorted, and is no good.

Tubular or paper condensers.
Figure 89.-Tubular or paper condensers.

The ELECTROLYTIC CONDENSERS are usually larger than tubular condensers. In figure 90, the one to the right with a large numeral 1 is about 6 inches long and 1½ inches in diameter. The others are of sizes in proportion to this one.

The plates of electrolytic condensers are tin foil, or a slightly thicker sheet metal. The dielectric is a porous paper soaked in CHEMICALS. The metal plate and dielectric are rolled together just as they are in tubular condensers.

All electrolytic condensers have a POSITIVE and a NEGATIVE terminal. When connecting one of these into a circuit, you must be very careful to connect the + on the condenser to the POSITIVE potential in your radio. Otherwise the condenser is liable to blow up or burn out.

Electrolytic condensers.
Figure 90.-Electrolytic condensers.
The VARIABLE CONDENSER is the gadget you turn when you TUNE your radio. There are a great many different sizes and shapes of these, but all present an appearance similar to those in figure 91. One set of plates is FIXED and the other MOVABLE. It will not be unusual to find

Variable condenser.
Figure 91.-Variable condenser.
two, three, and sometimes four sets of condensers GANGED together on a single shaft.

When you start digging into the chassis of a radio, don't be surprised to find condensers other than those described here, because someone is always dreaming up a new design. The schematic symbols for FIXED and VARIABLE CONDENSERS appear in figure 92.

Symbols for fixed and variable condensers.
Figure 92.-Symbols for fixed and variable condensers.


The condenser is the only known device that can store ENERGY as ELECTRICITY. In a battery, it is stored as chemical energy. In a coil, energy is stored in the magnetic field. But in a condenser, it is stored as DISPLACED ELECTRONS.

In the first chapter of this book, you were told that metals have a great many FREE ELECTRONS, and that when the proper force is applied these electrons may be pulled away from one metal and added to another.


In figure 93, the two plates of the condenser are designated as P1 and P2. In drawing A, the switch S is turned to position 1. The emf of the battery will pull FREE electrons off P1 and place them on P2. This CHARGES THE
Condenser action.
Figure 93.-Condenser action.
CONDENSER. Since electrons have been removed from P1 and added to P2, P1 will be POSITIVE and P2 NEGATIVE.

When switch S is moved to position 2, the condenser will DISCHARGE. Excess electrons will leave P2 and run back onto P1, until an EQUAL number of electrons are on each plate.

Each time the switch is turned to position 1, the condenser will charge, and when the switch is turned to position 2, it will discharge. This CHARGING and discharging is known as CONDENSER ACTION.

A condenser is FULLY CHARGED when the potential across the plates is EQUAL to the APPLIED emf. And it is completely discharged when the potential across the plates is ZERO.

A condenser does not become charged the instant The emf is applied. The amount of time required for a condenser


to charge or discharge is determined by two factors-the RESISTANCE of the circuit, and the SIZE of the condenser.


The SIZE of a condenser does not refer to its physical dimensions, but rather to the number of electrons that can be moved with one volt of applied emf.

The size of a condenser is more properly known as its CAPACITY. The unit of CAPACITY is the FARAD-the ELECTRICAL EQUIVALENT of ONE VOLT displacing a COULOMB of electrons.

A condenser with a capacity of one farad would be larger than a 5" 38 gun mount. The usual condenser capacity is in millionths of a farad-a MICROFARAD. Or still smaller, a MILLIONTH of a MILLIONTH-a MICRO-MICROFARAD. The abbreviation of microfarad is mfd or μfd, and of micro-microfarad it is mmfd or μμfd.

Mica condensers have capacities ranging from about 10 mmfd to 1,000 mmfd. Paper condensers range from 0.0001 mfd to five mfd, and electrolytic condensers from about one mfd to 50 or 60 mfd.

Many special condensers with capacities greater or smaller than those listed have been made, but the majority used in your radios are within the indicated ranges.

VARIABLE CONDENSERS also have a wide range of capacities, of course. AT FULL MESH-all the way closed-they vary from 400 mmfd down to about 10 or 20 mmfd.


One of the most used features of a condenser is its ability to BLOCK the flow of d.c., and to CONDUCT a.c.

When a condenser is connected into a d.c. circuit, it will quickly become charged. When fully charged, the flow of electrons is stopped, and the condenser acts as an OPEN CIRCUIT.

A condenser connected into an a.c. circuit acts quite differently. In figure 94, a condenser is connected in series with a lamp and a generator. During the positive


A condenser conducts a.c.
Figure 94.-A condenser conducts a.c.
half-cycle, electrons leave the upper plate, flow through the generator and lamp, and onto the lower plate of the condenser. But, during the negative half-cycle the electrons reverse their direction. Now they flow out of the lower plate, through the lamp and generator, and onto the top plate of the condenser.

Each time the cycle reverses, the electrons leave one side of the condenser and enter the other. In this way electrons continue to flow through the lamp as long as the generator is running.

Remember that electrons do not go through the condenser. They merely run out of one side and in at the other.


A condenser has an opposition to the flow of current, and like coils, the opposition is called REACTANCE-or more specifically, CAPACITIVE REACTANCE, XC.


The amount of capacitive reactance is determined by the CAPACITY of the condenser and the FREQUENCY of applied emf. With a condenser, the LARGER the CAPACITY and higher the FREQUENCY the LOWER the XC. The formula for finding the capacitive reactance of condenser is-

XC = 1 / 2πfC

Where XC is the capacitive reactance in ohms,
f is the frequency in cycles,
C is the capacity of the condenser in farads.
2π is 2 X 3.14 or 6.28.

Since both f and C are in the denominator, the LARGER they become the smaller XC will be.

A condenser with a capacity of one mfd carrying an a.c. of 100 cycles will have an XC of-

XC = 1 / (2 x 3.14 x 100 x .000001)
XC = 1600 ohms approximately

But, at a frequency of 1,000,000 cycles, it will have an XC of only-

XC = 1/ (2 x 3.14 X 1,000,000 X .000001)
XC = .16 ohms approximately

If you DOUBLE the capacity of a condenser you cut the capacitive reactance in HALF. Increasing the size of condenser by 10 reduces its XC, (at any specific frequency) to 1/10 of its former value.

Remember this, as the FREQUENCY of the current and CAPACITY of the condenser INCREASE, the CAPACITIVE REACTANCE of the condenser DECREASES.



Recently, a new style of condenser has been used in many of the high frequency circuits of receivers. The first glance at this newcomer would make you think it is a resistor, but careful examination would reveal the error of your guess. Look carefully when you see a device that looks like figure 95. It may be a new style condenser.

Ceramic condensers.
Figure 95.-Ceramic condensers.
These condensers are made of silver coated on a ceramic material. They are used to compensate for changes in capacity with temperature variations.

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