May 1934 Radio News and Short-Wave
of Contents] These articles are scanned and OCRed from old editions of the Radio & Television News magazine.
Here is a list of the Radio & Television News articles
I have already posted. All copyrights are hereby acknowledged.
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
Student's Radio Physics Course
Alfred A. Ghirardi Lesson 29
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 used.
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 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
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 the
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
As 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.
Sometimes 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
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