July 1932 Radio News
[Table
of Contents]
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 19191959. All copyrights hereby
acknowledged.

I've always had a problem with book and article titles containing
the word 'Modern' because it is utterly ambiguous. What was
modern in 1932 is usually obsolete merely a decade later, especially
in high technology (not so much in buggy whip stateoftheart
methods, though). Sometimes, as with this article on insulation
breakdown voltages, bringing the information up to date requires
only the substitution of a few words. For instance, replace
'condenser' with 'capacitor' and units of 'mfd' with 'μF'
and 'mmfd' with 'pF,' then you'll be on your way to gaining
useful information. There is a nice nomograph for use in designing
capacitors for specific voltage handling and a table of dielectric
puncturing voltages as well.
Orion Instruments has a very extensive table of
dielectric constant values.
Modern Radio Practice in Using Graphs and Charts
By John M. Borst
Part Seven
Calculations in radio design work usually can be reduced
to formulas represented as charts which permit the solution
of mathematical problems without mental effort. This series
of articles presents a number of useful charts and explains
how others can be made
Dielectric Strength
Figure 2  Table of breakdown voltages for various types of
sheet insulation.
The capacity of a homemade condenser is often more or less
of a mystery. The amateur or experimenter who does not possess
a bridge or capacity standard must calculate the capacity. Conversely,
if a condenser of a given capacity is desired, only a calculation
will eliminate guesswork.
The standard formula has been transformed into an alignment
chart in Figure 1. The capacity of a condenser can be found
when the area of the plates, their number, distance and the
kind of dielectric are known.
The relation between centimeters and inches or mils as well
as the relation between square centimeters and square inches,
centimeters and microfarads is also shown in Figure 1. The "dielectric
constant," also called "inductivity" or "specific inductive
capacity," is incorporated on the chart, which makes the consultation
of any sources superfluous.
The formula for the capacity of a condenser consisting of
parallel plates is
in micromicrofarads
where A = the area of one plate in square centimeters
d = the distance between
two plates in centimeters
n = the number of plates
K = the specific inductive
capacity
This expression refers to a condenser with alternate plates
in parallel. The formula does not take into consideration the
spreading of the lines of force at the edges of the plates.
This effect is negligible so long as the thickness of the dielectric
is small compared to the area of the plates.
In designing this chart the prime idea has been to cover
all possible cases which occur in practice. Therefore, the capacity
scale ranges from 1 micromicrofarad to over 10 microfarads,
and the other quantities also cover a wide range.
Examples
Two metal plates have an area of 1 square inch and are placed
parallel, 1/4 inch apart, in air. What is the capacity?
Referring to the chart, draw a line from the 1squareinch
mark on the "Area" scale to 1 on the K scale. The specific inductive
capacity of air is one (unity). This gives you an intersection
on the turning scale No. 1. From this newly found point draw
another line through the point 2 on the N scale and find a second
point on the turning scale No.2. The final line is drawn through
the latter point and the 250mils mark on the d scale. This
line intersects the capacity scale at 0.9 mmfd.
When exactly 1 mmfd. is required, the last line should
be turned around its point on the turning scale No. 2 until
it intersects the capacity scale at the 1 mmfd. mark and
the intersection on the d scale shows the required distance
between the plates (225 mils). The distance, however, can
be left the same and the problem worked backwards, in which
case an area of 1.1 square inch is found necessary. These lines
have not been added in Figure 1 because they are so close
together that it might confuse the reader.
When using these charts. needless to say, one should not
actually draw the lines but use a transparent ruler, a regular
ruler or a tight thread.
The second example shows how to work the problem backward.
Suppose a paper condenser of 1 mfd. is wanted and the dielectric
available has a thickness of 2 mils. This is manilla paper,
treated with paraffin. Its specific inductive capacity is 3.65
and the breakdown voltage may run as high as 250 volts per
mil. There is one more quantity which can be chosen and then
the other one is determined. This can be either the number of
plates or the size of the plates. The number of plates is the
best to assume, because this has to be a whole number. Let us
assume there shall be 30 plates.
For the solution of this problem, start at the 1 mfd. mark
on the capacity scale. A line from this point to the 2 mil.
mark on the d scale intersects the turning scale No. 2. Draw
a line through the latter point and through the point representing
the number of plates (30). Now note the intersection on the
turning scale No. 1. Finally draw the last line from the point
representing the dielectric constant. 3.65, through the point
on the turning scale No.2, which shows the necessary area of
the plates as 84 square inches. As a checkup, an actual calculation
gave the area as 83.7 square inches.
The experience of this second example teaches us that in certain
cases the last line would intersect the area scale beyond the
limits of the paper. This means that the area of the plates
needed is going to be larger than 100 square inches. If the
area is to be smaller than 100 square inches, either the number
of plates have to be increased, the thickness of the dielectric
decreased or the material exchanged for one with a greater inductivity.
Then try again.
If one wishes the problem solved for values of variables
outside the range of the chart, then some multiplying stunt
has to be employed. For instance, suppose the paper in the above
example had been dry paper with a dielectric constant of 1.8,
then the last line does not intersect the area scale within
the limits of the page. Therefore, multiplying 1.8 with any
convenient number  say, 5  the last line is drawn from 9 through
the intersection on the turning scale number one and the area
scale is intersected at 34.
This result must now be multiplied by five in order to find
the correct answer, which is 170 square inches.
While determining the specifications for a condenser it is
important to be sure that the dielectric will stand the applied
voltage. Therefore a list of the breakdown voltages for different
materials is found in Figure 2.
Capacity of a Condenser (aka Capacitor)
A Chart (Nomograph) That Works For You
Figure 1  The size of condenser plates their distance apart,
the number of plates, kind of dielectric or capacity can be
found from this chart if the other four quantities are known.
The five quantities are on three straight lines as shown in
the example above.
Temperature influences the ability of a dielectric to withstand
electric pressure. When the condenser heats up under a continuous
load, the breakdown voltage is lowered. Therefore the tests
of such condensers must be made over a considerable time at
working voltage or at a much higher voltage for a short time.
Commercial paper condensers usually consist of long strips
of prepared paper, with tinfoil interleaved, which is then rolled.
In the case of rolling a condenser with an even number of plates,
the top plate and the bottom plate form an additional section
of the condenser so that in this case the rolling has the effect
of adding one more plate. The reader should see whether the
dielectric for this additional section has the same thickness
as the other sections and make allowances for any possible difference.
When the number of plates is odd or when the paper is not
rolled, the actual number of plates is used for the calculation.
The accuracy of a calculation by means of this chart will
be sufficient only if the correct values for the dielectric
constant and the thickness of the dielectric have been determined.
This is sometimes difficult to accomplish, especially with paper
as a dielectric. If the reader guesses at the constant and the
actual separation of the plates, he must expect the result to
be off accordingly.
Nomographs Available on RF Cafe:

Decibel Nomograph

Voltage and Power Level Nomograph
 Voltage, Current, Resistance,
and Power Nomograph
 Resistor
Selection Nomogram
 Resistance
and Capacitance
 Capacitance
Nomograph
 Earth Curvature Nomograph
 Coil Design Nomograph

Coil Inductance Nomograph
 Antenna Gain Nomograph

Resistance and
Reactance Nomograph
Posted September 10, 2013