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 Gauge
April 1932 Radio News
of Contents]These articles are scanned and OCRed from old editions of the Radio & Television News magazine.
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vintage Radio News
The Radio Physics Course: Resistances in Radio
Lesson Nine (Continued from Lesson Eight)
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 class assignments.
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.
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,
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
L = length
of conductor in feet.
C.M. = cross section area of conductor in
a = temperature coefficient of the material per degree
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
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 Equipment
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
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.
3. 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?
5. From 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.
Describe the construction of the vitreous enameled type of wire-wound
7. Describe the construction
of two forms of high resistors used in radio receivers in places where
very little current will be flowing.
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?
9. Describe the construction
of a variable wire-wound resistor.
installation conditions affect the power in watts which a resistor can
11. Draw a symbol for (a) a fixed resistor, (b) a
variable resistor, (c) a resistor tapped at the middle, (d) a resistor
tapped at three places.
General Quiz on This Issue
1. How do strong ultra-short radio
waves affect the human body?
2. What is a
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 this
6. Explain why a Stenode receiver tends
to reduce background noise.
7. How are the
grids placed, in one of the newest tube designs, to make the tube characteristic
identical when either grid is in the circuit?
Why does this tube lend itself particularly well to automatic volume
9. What forces of conditions may
upset the normal balance of charges in the atom?
What are the advantages of incorporating one i.f. stage in a superheterodvne
type short-wave converter?
Principle, commonly used in measuring resistance, is employed im one
type of simplified remote control, and how is this principle utilized?
12. How may signals of high frequencies be
concerted to signals of lower radio frequencies?
How may two ribbons of silk be made to generate 1,000,000 volts?
14. What are the advantages of the r.f. pentode
*Radio Technical Pub. Co., Publishers Radio Physics
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