The Radio Physics Course: Resistances in Radio
April 1932 Radio News Article
heard of 'Litz' wire, right? It's the twisted bundle of multiple
enamel or otherwise coated wire used for making couplers, antennas,
and at frequencies up to about a couple MHz. Congratulations,
but did you know the full name for it is 'Litzendraht?'
Neither did I until after reading this article. 'Litzendraht' is
the German word meaning 'braided wire' or 'woven wire.' Litz by
itself means braided or woven. So, technically if you call it Litzendraht
wire, you are being redundant since it is the same as saying woven
wire wire. That might save you some embarrassment one day if you
happen to be working around a German techie. Litzendraht is used
in order to exploit the skin effect at high frequencies where the
majority of the current is conducted on the wire's surface. Using
multiple insulated wires enables greater current carrying capability
than an equivalent diameter single solid wire. While dispensing
trivia, note also the use of the term 'B. & S.' gauge for wire.
It stands for
Brown and Sharpe, which is the equivalent of the American Wire
April 1932 Radio News
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.
The Radio Physics Course: Resistances in Radio
Lesson Nine (Continued from Lesson Eight)
in Radio, Resistance alloys, How the resistance units are actually
made and used .
This series deals with the study
of the physical aspects of radio phenomena. It contains information
of particular value to physics teachers and students in high schools
and colleges. The Question Box aids teachers in laying out current
By Alfred A. Ghirardi*
Since the temperature of a conductor may be changed by the weather
conditions or by the heat developed in the wire itself due to the
passage of current through it, the temperature must be taken into
account when calculating the resistance if accurate results are
desired. The resistance of pure metals and most alloys increases
as the temperature rises. The resistance of carbon and electrolytes
(fluid conductors) decreases as their temperature rises. The amount
of change of resistance varies with the different conductors, but
for pure metals the increase in resistance is nearly 0.4% for each
change of one degree Centigrade.
Figure 1. Shows the stages in building
up this form of resistance. At the bottom is the porcelain
tube upon which is wound a resistance wire shown at center.
At top is the enamel coating which is then baked on after
the terminal has been attached.
Manganin is an alloy of
84% copper, 12% nickel and 4% manganese, developed especially for
use in the shunts of ammeters and for precision resistances. Therlo
is a similar alloy. Its change in resistance per degree is one part
in 100,000. "Constantin" is another alloy whose resistance does
not change materially. It consists of approximately 60% copper and
40% nickel. It is used in rheostats and measuring instruments.
The amount in ohms that a piece of the material having a
resistance of one ohm changes for each change of one degree in temperature
is known as the temperature coefficient of resistance ("a"). Thus
if a conductor has a resistance of one ohm at 20° C temperature,
it will have a resistance of one ohm plus the amount equal to this
coefficient at 21° C At 19° C it would have a resistance
of one ohm minus the coefficient, etc.
The average temperature
coefficient between 0° and 100° C and 32° and 212° F.
is roughly the same for all pure metals and is about 0.004 per degree
Centigrade, or 0.0023 per degree Fahrenheit (since one degree C
represents a larger change in temperature than one degree F).
The temperature coefficient for annealed copper is 0.00210
at an initial temperature of 68° F (on the Fahrenheit scale),
or 0.00377 at an initial temperature of 20° C (on the Centigrade
scale). The value of the temperature coefficients of the various
resistance alloys used in radio work for winding fixed or variable
resistors must be obtained from the manufacturers of the resistance
wire in any case when exact calculations are to be made.
Since the specific resistance of the conducting materials is
usually given for the material at standard temperature of 20° C,
the formula must be altered if we are to take into account the change
of resistance due to the fact that the conductor may be at a temperature
above or below 20° C in actual practice. To calculate the
true resistance of any metallic conductor at any temperature (up
to 100° C) use the formula
where R = resistance of the conductor in ohms at operating temperature.
k = specific resistance of the conductor at 20° C
= length of conductor in feet.
C.M. = cross section area of
conductor in circular mils.
a = temperature coefficient of the
material per degree C.
t = difference in degrees between the
operating temperature and the standard temperature at which the
specific resistance k is specified (20° C in most cases).
The ± sign inside the bracket means that if the temperature
of the conductor is above the standard of 20° C. the resistance
increases so the plus sign is used. If the temperature is below
20° C the resistance is less and the minus sign is used.
Example: A piece of No. 18 B. & S. gauge copper wire
600 feet long is wound up to form a circular field coil for an electro-dynamic
loudspeaker. When the normal current flows through the coil its
temperature rises to 60° C What is the exact resistance of
the coil during normal operation?
Solution: From the copper
wire table we find that a No. 18 wire has a cross-section area of
1620 circular mils. The specific resistance of annealed copper is
10.35 at 20° C. Its temperature coefficient is 0.00377 per
degree C. "t" in the formula is therefore equal to 60 - 20 = 40
degrees. Substituting these values, we obtain
from which R = 4.37 ohms. Ans.
Resistors in Radio
There is naturally a certain amount of resistance in every electrical
circuit due to the resistance of the connecting wires, joints, contacts,
etc. The resistance of a circuit can be kept low by making it as
short as possible, using a good electrical conductor (such as copper),
and making its cross-section area large. (Due to the fact that very
high frequency currents travel only through a thin surface layer
of the wire, "skin effect," wires for conducting this type of current
are often made up of a number of very small conductors insulated
from each other by an enamel, cotton or silk covering. (This is
called Litzendraht wire.)
Wire-wound Resistors of Various Types
Figure 2. Here
are shown several forms of resistors of the vitreous enamel
types, the smaller ones with screw terminals attached. The
two lower resistors are equipped with the standard Edison
lamp socket bases. At right, second from top, is a completed
resistor similar to that shown in Figure 1. The resistor
H contains a number of taps that make it suitable for voltage
divider work. It is equipped with mounting legs.
In radio equipment resistance
is purposely introduced at various places in the circuits in order
to reduce or control the amount of current flowing, reduce the effective
voltage applied to a device, or cause differences of potential which
are utilized for some definite purpose (C bias resistors), etc.
A resistor is a device whose purpose is to intentionally provide
resistance in an electrical circuit. Resistors may be made either
fixed or adjustable (variable). Fixed resistors are those whose
value cannot be changed readily while in use. Adjustable resistors
may be varied in value. Fixed resistors are used in the filament
circuits of battery-operated vacuum tubes, in the voltage dividers
of B eliminators, for leaks, resistance couplings for furnishing
grid or C bias voltages, etc. Variable resistances are not used
as much in radio receivers nowadays as they formerly were, due to
the tendency to eliminate as many control knobs from the panels
as possible. They are still employed as rheostats, potentiometers,
volume controls, etc.
enameled resistors are used extensively in power packs of radio
receivers. They are made by space-winding the resistance wire on
a special porcelain tube base. The base, including the terminal
connections, is then coated with a powdered glassy enamel and fired
at red heat. The result is a resistor unit covered with a vitreous
enamel coating which protects the fine resistance wire from mechanical
injury and serves as an excellent heat conductor to rapidly conduct
the heat from the resistive element to the outside surface. This
construction permits the finest resistance wire to be used without
danger of oxidation or other chemical depreciation. The enamel also
holds the resistance wire in place without any mechanical strain,
and no strains can be set up by heating or cooling, as the vitreous
enamel and the wire expand and contract together. Figure 1 shows
a resistor of this type during the various stages of manufacture,
from the bare porcelain base tube at one end to the completely vitrified
resistance winding at the other end. This is a voltage divider resistance
used in power packs. Special resistance wires made from alloys of
nickel and iron have been developed for winding these resistors.
They have very low temperature coefficients of resistance and therefore
their resistance does not change very much when they get warm in
service. Several resistors of this type made up in special forms
for use in radio receivers are shown in Figure 2. Resistor H is
variable in value in steps.
Physics and science instructors will find these
review questions and the "quiz" questions below useful as reading
assignments for their classes. For other readers the questions provide
an interesting pastime and permit a check on the reader's grasp
of the material presented in the various articles in this issue.
The "Review Questions" cover material in this month's installment
of the Radio Physics Course. The "General Quiz" questions are based
on other articles in this issue as follows: The "Twin-Grid" Tube,
A Modern Quartz-Crystal Receiver, Latest Short-Wave Converter, With
the Experimenter, Phenomena Underlying Radio, A 16.5 to 550-Meter
"Super" of Radical Circuit Design, The March of Television, Radio
Fever, Two New Tubes.
1. Calculate the power supplied
to the filament of a 280 type rectifier tube which takes a current
of 2 amps. at 5 volts.
2. State the four
factors upon which the resistance of a conductor depends and explain
just how each one affects the resistance.
The diameter of 1000 ft. of No. 24 B. & S. copper 'wire, used
[or the winding on a filament transformer is .0201 inches. What
is its diameter in mils? What is the circular mil area? If the specific
resistance of copper wire is 10.35 at 20° C, what will be the
resistance of this wire at a temperature of 20° C?
4. What is the resistance of the wire in problem
3 at all operating temperature of 80° C if the temperature
coefficient of copper is 0.004?
the table of specific resistances of carious materials in your book,
write down the ten metals having the highest specific resistances.
Next to each, write down how many times greater its resistance is
than that of annealed copper.
the construction of the vitreous enameled type of wire-wound resistor.
7. Describe the construction of
two forms of high resistors used in radio receivers in places where
very little current will be flowing.
Describe the construction of a variable high resistor designed to
carry a small amount of current without overheating. What is the
purpose of the flaked mica in this?
Describe the construction of a variable wire-wound resistor.
10. What installation conditions affect the
power in watts which a resistor can dissipate?
a symbol for (a) a fixed resistor, (b) a variable resistor, (c)
a resistor tapped at the middle, (d) a resistor tapped at three
General Quiz on This Issue
1. How do strong ultra-short radio waves
affect the human body?
2. What is a harmonic
3. What is the electrophorus?
4. What does the term "definition" mean
as applied to television? Why is it important?
The use of a quartz-plate circuit in a superheterodyne i.f. amplifier
admittedly attenuates the higher audio frequencies. By what method
is this attenuation overcome in the latest design of receivers of
6. Explain why a Stenode receiver
tends to reduce background noise.
are the grids placed, in one of the newest tube designs, to make
the tube characteristic identical when either grid is in the circuit?
8. Why does this tube lend itself
particularly well to automatic volume control?
What forces of conditions may upset the normal balance of charges
in the atom?
10. What are the advantages
of incorporating one i.f. stage in a superheterodvne type short-wave
11. What Principle, commonly
used in measuring resistance, is employed im one type of simplified
remote control, and how is this principle utilized?
How may signals of high frequencies be concerted to signals of lower
13. How may two ribbons
of silk be made to generate 1,000,000 volts?
What are the advantages of the r.f. pentode tube?
Technical Pub. Co., Publishers Radio Physics Course.
Posted November 1,