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May 1932 Radio-Craft
 [Table
of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
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This might be one of the earliest
printed instances of
Harold
A. Wheeler's simplified formulas for the three basic inductor forms. Wheeler
is credited with having devised the first automatic volume control (AVC) using
diode envelope detection. We all
use them on a regular basis, but for most the origin was never known or has long
since been forgotten (I fall into the latter category).
I did some research on Wheeler's inductance formulas a few months ago while working on what is
now titled "RF
Cafe Espresso Engineering Workbook™," so it
was sort of déjà vu when this blurb appeared in a 1932 edition of Radio-Craft magazine.
Simplified Coil Calculation
Inductance Calculation Dimensions
By G H. W. Nason
The archaic method of calculating inductances involves a formula taking into
account not only the actual dimensions of a winding and the number of turns of wire,
but a form factor "K" dependent upon the ratio of length to diameter of the form
on which the coil is wound. (See page 109, August 1931 issue.) While these formulas
are no doubt, accurate to a minute degree in capable hands, the errors possible
are manifold; and rarely, if ever, does a coil so designed come within a reasonable
degree of the desired inductance.
A considerable simplification of the design problem was evolved several years
ago by Harold A. Wheeler of the Hazeltine Laboratories, who is responsible also
for the multiplex detector and automatic volume control used by Philco, Fada, and
other Hazeltine licensees.
In the illustrations, herewith, three types of winding-so which cover practically
every case within the needs of the experimenter or Service Man in his daily work
are shown. First, we have a multi-layer winding, such as might be employed in the
intermediate-frequency transformers of a superheterodyne receiver. Second on the
list is a simple solenoid of the type used in tile tuned circuits of broadcast receivers.
The last is a helical (spiral) winding such as might be used either as a coupling
coil in a band selector, as an antenna coupling coil, or as a primary winding for
an R.F. transformer. The equations for calculating the inductance are given with
each sketch. All dimensions are to be taken in inches, and the answer will be obtained
directly in microhenries.
The method compares quite favourably with Nagaoka's formula as to accuracy, and
is many times easier to use than the older method, in which the form factor had
to be taken into account. Accuracy to 1% is obtainable in the case of the multi-layer
coil, when the three terms in the denominator (below the line) are nearly equal.
The accuracy in the case of the simple solenoid is also to 1% when the length of
the winding is greater than four-fifths times the diameter. In the third case. this
degree of accuracy is obtainable when the dimension "c" is greater than one fifth
the dimension "a".
In no case will the error be greater than is possible with the more tedious method
formerly used, when the most exacting care is taken. All that is necessary for the
calculation of inductance values is a ruler, a pencil and a copper wire table giving
the diameter of various wire sizes, so that the space occupied by a given winding
may be known. (See page 186, September 1931 issue).
Posted August 28, 2023
(updated from original
post on 8/31/2015)
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