Student's Radio Physics Course
May 1934 Radio News and the Short-Wave
the equation for the inductance of a standard tightly-wound, single-layer
inductor is very easy. What, though, if you needed to determine
how to build an iron-core inductor or to build a choke coil with
a silicon steel laminated transformer-iron core where the windings
have an air gap between them? Who ya gonna call? Alfred Ghirardi,
of course, or at least his ghost through this article from a 1934
edition of Radio News and the Short-Wave. If you also need
advice - complete with drawings - on how to wind a coil that will
not induce a killer counter-EMF when a switch is opened (as with
a solenoid), then here, too, is your source.
May 1934 Radio News and Short-Wave
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.
Student's Radio Physics Course
Alfred A. Ghirardi
The inductors used in radio apparatus take many
forms, depending on their particular application. Some have inductances
of only a few microhenries, are wound on insulating forms, and have
air cores. Broadcast frequency tuning coils may be of the order
to 200 to 300 microhenries and may be from 1 to 3 inches in diameter,
wound with 50 to 100 turns or so of No. 20 to No. 30 wire. Radio-frequency
choke coils having an inductance of 85 millihenries are also used
extensively. For shortwave work, smaller values of inductance are
Iron-core inductances commonly used in audio amplifiers
and in the filters of the B power-supply units of radio receivers
have a great many turns of fine wire wound on laminated steel cores.
Inductances as high as 100 henries are not uncommon in devices of
this kind. The windings in inductances as large as this contain
thousands of turns of wire. Their particular applications will be
studied later. The approximate inductance of iron-core inductor
or choke coils built with silicon steel laminated transformer-iron
cores may be calculated from the following formula:
A core flux density of 20,000 lines per square inch is assumed.
The inductance is in henries, the core cross-section area
is in square inches, and the total air gap in inches is determined
from the formula.
The size of wire with which to wind the coil is determined
by the current the coil is to carry. The wire size may be obtained
from the data in a magnet wire table.
When an alternating
current, or a varying direct current, flows through an inductive
winding, a considerable counter-e.m.f. is developed due to the varying
magnetic flux. This acts to oppose the flow of the current through
the winding, as we shall see later.
The large spark noticed
when opening the switch in an inductive circuit is caused by the
high self-induced voltage which tends to keep the current flowing
across the switch gap. Circuits having high inductance should not
be opened suddenly, for dangerously high voltages may be developed
in them by the self-induction. These may be high enough to puncture
the otherwise satisfactory insulation on the wires. Circuits of
this kind should be opened gradually by inserting resistance in
them to slowly reduce the current to a low value, then finally opening
In some applications of coils where wire is
wound up in the form of a solenoid in electrical and radio work,
it is desirable that the solenoid should not possess any appreciable
amount of inductance. Such windings are called non-inductive windings.
For instance, when resistors are made of resistance alloy wire,
the wire is usually wound up in the form of a solenoid of many turns,
in order to make it compact in size. It is often desirable that
the resistor not have any appreciable inductance due to this wound
form, as in this case of the resistor coils used in Wheatstone bridges.
Self-inductance in a coil may be neutralized by winding
one-half of the coil in one direction and the remainder in the opposite
direction as shown at (A) in Figure 1. The wire is really doubled
back on itself. This is accomplished by folding the length of wire
to be used, at its middle point, and starting at this point, winding
both halves at the same time as a single wire, until the ends or
terminals are reached. The magnetic effects of the current flowing
in one direction through half of the total turns is equal and opposite
to that produced by the same current flowing in the opposite direction
through the other half of the total turns. The magnetic fields thus
neutralize each other, and hence no inductive effect is present.
The winding is said to be non-inductive.
this method is rather inconvenient when a long length of wire is
to be wound up, etc., it is common in manufacturing non-inductive
coils or windings to simply wind two wires side by side and join
the ends, or to wind the total wire up in the form of two separate
coils, each having an equal number of turns equal to half the total
turns required, as shown at (B) and (C) of Figure 1, instead of
in a single part. Then the proper ends of the coils are connected
together as shown, so the current progresses from one end through
the two coils in the opposite direction so the magnetic fields are
neutralized. The coils need not be wound in the same direction.
It is merely necessary to connect them properly so the current flows
in the opposite direction in each. At (B) the coils are wound similarly.
At (C) they are wound in opposite directions.
the inductive effect of one coil is neutralized by current sent
through a separate "bucking winding" of the proper number of turns,
placed near it as shown at (C) of Figure 1. The bucking coil is
so wound or connected that its magnetic effect equals and opposes
that of the main field. In these methods, the two windings need
not be in the same direction. The right-hand rule for the magnetic
field of solenoids is employed for working out the proper current
Another way of winding a coil that is almost
non-inductive is to wind it flattened in shape on a thin flat cardboard
or Bakelite form about J1/8 inch thick. Such a coil has practically
no inductance because the opposite sides of each turn of wire are
so close together that the magnetic fields neutralize, since the
current is flowing in the opposite direction in them as shown in
(E) of Figure 1.
Posted February 19, 2014
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