October 1947 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.
This first installment in a
multi-part series on magnetism appeared in the October 1947 issue of Radio-Craft
magazine. At the time, there was great interest in magnetic tape and even
magnetic wire recorders. Lots of articles were published on the electronics and
mechanics of recorders, but relatively few discussed the physics of magnetics.
Although the title says it is about permanent magnetic theory, there is also a
lot of information on electromagnets. Terms such as reluctivity, magnetomotive
force, magnetic flux, conductivity of electrical and magnetic circuits,
conductance, B-H curves, maxwells, oersteds, and gilberts are introduced, along
with some simple equations relating everything. Part II, "Benefits of Tape
Recording," appeared in the November 1947
issue and Part III, "Magnetic Recording - Recorder
Design," appeared in the December 1947 issue.
Magnetism - Part I - A modern view of permanent magnet theory
By A. C. Shaney
Fig. 1 - Illustrating the new magnet markings.
Fig. 2 - Simple equipment for magnet studies.
The subject of magnetism is very old (the compass appears to have been invented
over 4,000 years ago), yet it remains one of the least understood of phenomena.
It was not until the twelfth century A.D. that European scholars tried to fathom
its secrets, though the magnet or lodestone had been known to them for centuries,
and mention of it is found in legends dating several centuries B.C.
In Europe as in Asia the magnet found its almost exclusive use as a means for
determining direction. This unfortunately caused its poles to be given geographical
names (north and south). This clumsy terminology has been an obstacle to the student.
We will therefore rename the poles arbitrarily, calling the north (N) pole the positive
pole and the south (S) pole the negative pole. To avoid confusion with electrical
signs, magnetic pole signs will be circled in drawings, as shown in Fig. 1, and
put inside parenthesis in text. This is one of a few new concepts we will introduce
in an attempt to present magnetic phenomena more clearly to technicians and experimenters
who may be interested in contributing to the art of magnetic recording.
New Magnetic Concepts
Probably nearly every reader (including the author) was first introduced to the
mysteries of magnetism via the old familiar bar magnet. When, in 1820, Sir Humphry
Davy magnetized a bit of soft iron by running a current of electricity around it,
he produced the first bar electromagnet. For an unobstructed understanding of magnetic
recording it is suggested that the reader temporarily forget Davy's bar magnet and
how it was made. To help forget, try this experiment:
1. Disassemble an old transformer made with E-1 laminations.
2. Stack all the E's together.
3. Slip a coil of wire (about 100 turns of No. 20 A.W.G.) over the center leg.
4. Place a steel bar across all 3 legs as in Fig. 2.
5. Apply 6 or more volts d.c. to the coil for a few seconds.
6. Pull the steel bar away from the laminations.
7. Examine the polarity of this bar magnet with a compass and by sprinkling powdered
iron on a sheet of paper placed over the magnet.
Fig. 3 - Two types of permanent magnet fields.
Fig. 4 - Electric analog of multipole magnet.
Fig. 5-a - Circuit. for making a disc magnet. 5-b - Chart of
disc magnet's force field.
Fig. 6 - Magnetization of a strip of steel.
Your examination will show that you have made a magnet with like poles at both
ends! The powdered-iron pattern will look like Fig. 3-b instead of the usual magnetic
field pattern of Fig. 3-a.
This unusual type of magnet is one of the cornerstones of modern magnetic recording
Two important things should be remembered about this magnet:
1. It was formed without passing an electric current around it.
2. Its polarity resembles that of 2 common bar magnets which have been physically
joined at like poles.
If this twin magnet be bent into the familiar horseshoe form, it will not exhibit
usual magnetic properties at its terminal poles.
This magnetic characteristic resembles the effect produced by connecting 2 similar
batteries back to back and measuring "no voltage" across their outermost terminals
If we should pass some current around the flat surfaces of a hard steel disc
(Fig. 5-a) about 1 inch in diameter and 1/4 inch thick, we would have a short cylinder
magnetized in the direction of its diameter (Fig. 5-b). (You should have forgotten
about ordinary bar magnets by now.) If a hole is drilled through its center, a shaft
inserted and the wheel rolled on a small hard steel rod or rail (Fig. 6), the rail
will become weakly magnetized at the points of polar contact. This passage of magnetic
induction from one magnet into a magnetizable substance by contact (or near contact)
and the retention of the magnetism by the latter is known as transference. Susceptibility
to transference determines the degree of interference to be expected when a turn
of magnetically recorded wire comes in direct contact with an adjacent wire.
We now have a rail (a magnetic signal carrier) which has been magnetized (modulated
by a magneto-mechanical modulator). We can expose or view (detect) these magnetic
modulations in a number of ways. A small compass may be used to explore the magnetic
field around the rail; or finely powdered iron may be sprinkled on the rail to find
its magnetic nodes; an electro-magnetic detector (a soft iron horse-shoe-shaped
rod which passes through a multi-turn coil connected to a sensitive galvanometer
and whose ends are separated by a distance equal to half the circumference of the
magnetic disc), as illustrated in Fig. 7 may be moved along, and in contact with,
the rail; or the rail may be moved against the detector. In any case a voltage will
be produced in the coil and indicated by the galvanometer.
This electromagnetic detection process may be reversed by applying a variable
voltage to the coil while the carrier (rail) is moved laterally. The rail will be
magnetically modulated as suggested in Fig. 2.
We now have covered the 3 basic elements required to record or transmit intelligence
via magnetic media:
1. Magnetic modulator (impresses magnetic modulation on a magnetic carrier);
2. Magnetic carrier (carries the magnetic modulation);
3. Magnetic detector (detects magnetic modulation in the carrier).
As the construction, application, and operation of these basic elements play
indispensable roles in the final processes of magnetic recording it becomes important
to be able to design these elements for optimum performance. Since they are all
primarily magnetic devices, designing them naturally calls for the use of magnetic
terms. It is therefore important for a full understanding of the process, to become
familiar with applicable magnetic terminology, bearing in mind the very special
applications to which it will be applied.
Familiarity with academic magnetic terms may be gained by referring to one of
a number of text books on magnetism. (Also see "Coils, Cores and Mag-nets" Radio-Craft,
October and November, 1946.) For the radio technician and experimenter, it may be
easier to understand magnetic phenomena by referring frequently to electrical analogies.
Interpretation of technical descriptions requires an understanding of carefully
chosen units and standards to which the names of famous scientists have been applied
(for example, fundamental electrical units were named after scientists like Count
Alessandro Volta, Georg Simon Ohm, Andre Marie Ampere, James Watt, and others).
Fig. 7 - Simple magnetic modulation detector.
Table I - Magnetic Flux Density Conversion Relations
Table II - Magnetomotive Force Conversion Relations
The fundamental units of and formulas relating to magnetic phenomena as developed
by physicists are usually expressed in the metric (also called the centimeter-gram-second
or c.g.s) system, which has been internationally employed. Interpretation and application
of this information is made difficult for English-speaking students accustomed to
the clumsier, inch-ounce-second or foot-pound-hour system. Conversion into standard
English units are helpful. See Tables I and II.
In magnetics, as in other physical sciences, a fundamental force - Gilbert (after
William Gilbert) - is required to produce a quantity - Maxwell (after James Clerk
Maxwell) - in a given medium. The ability of the medium to help or hinder the flow
of the quantity determines its reluctivity (magnetic resistance) or permeability
Where Where R = resistance in R = reluctivity, ohms, H = magneto- E = electromotive
motive force force in volts, in oersteds, I = current in B = magnetic flux
amperes. in gauss. Similarly, the conductivity of electrical and magnetic circuits
may be approximately compared as follows:
When the force Gilbert is simply related to lineal length of the magnetic circuit
(1 Gilbert per centimeter), we have a unit of magnetizing force per unit of
length called an oersted (after Hans Christian Oersted). Similarly, when the quantity
maxwell is simply related to cross-sectional area (1 maxwell per square centimeter),
we have a unit of magnetic quantity-per unit of area - a gauss (after Karl F. Gauss).
The familiar Ohm's Law of electricity now can be compared with its magnetic'
equivalent as follows:
R = E/I
R = resistance in ohms,
E = electromotive force in volts,
I = current in amperes.
R = H/B
R = reluctivity,
H = magnetomotive force,
B = magnetic flux in gauss.
Similarly, the conductivity of electrical and magnetic circuits may be approximately
compared as follows:
G = E/I
G = conductance in mhos,
E = electromotive force in volts,
I = current in amperes.
M = B/H
M = permeability,
B = magnetomotive force in oersteds,
H = magnetic flux in gauss.
The Magnetic Carrier
Although many factors are to be considered in the design and manufacture of magnetic
tapes or wires for recording purposes, the most important are the residual induction
and coercive force of the material. These 2 characteristics are best understood
by examining the material's so-called hysteresis loop (which is nothing more than
a visual indication of the lagging of magnetic induction - flux - behind the magnetizing
force which produces the magnetism in the material), See Fig. 8.
Fig. 8 - Magnetic hysteresis, or B-H curve.
Fig. 9 - The important demagnetization loop.
Figs. 10-a and 10-b - Patterns with large gap.
Figs. 10-c. 10-d - Same signals, smaller gap.
The horizontal axis H represents the value of the magnetizing force per unit
length applied - positive and negative. The vertical axis B represents the amount
of flux per unit area induced in the material. Measurement begins with the material
completely demagnetized (at 0 intersection of lines H and B). The magnetizing force
is increased to a positive value beyond which further increases in magnetizing force
produce no increase in the magnetism of the material. This registers a curve from
0 to B max on the chart. Then the magnetizing force is reduced to zero. However,
a certain amount of induced magnetism remains in the material - as represented by
the intersection of the return curve from B max to B at Br. This Br value represents
the peak residual magnetism the material will retain without demagnetizing influence.
From this point, the magnetizing force is changed in direction and increased to
a negative saturation. This describes a curve from Br to an intersection with the
H line at - Hc and on to a negative B max. Again the magnetizing force is reduced
to zero, producing the curve from negative B max to negative Br. Re-establishment
of positive magnetizing force carries the curve from negative Br to positive B max.
Repetition of this magnetization and demagnetization process will result in the
establishment of the complete symmetric loop about point 0 - the hysteresis loop.
Most of the information required about a material is gained in the segment of
the hysteresis loop (Br to Hc) known as the demagnetization curve (Fig. 9). As explained,
the value marked on the B line at point Br indicates the maximum residual induction.
The value marked on the negative side of the H line at Hc indicates the demagnetization
force per unit length required to reduce the residual magnetism to zero. This is
known as the coercive force of the material. Furthermore, it can be shown that the
product of the flux density B and the unit demagnetizing force H represents the
amount of magnetic energy that each cubic centimeter of the material is capable
of supplying. In actual units, B x H divided by 8 π equals ergs per cubic centimeter.
In normal practice, however, only the product of B x H is quoted.
The variation of B x H between Br and Hc is shown on the right side of the demagnetization
curve. From a practical standpoint, it can be shown that a magnetic wire or tape
designed to operate at a point on the demagnetization curve corresponding to the
maximum value of B x H will supply the maximum amount of flux per unit volume of
Other important characteristics of magnetic signal carriers include factors which
influence transference, penetration, and self-demagnetization.
Transference, as previously indicated, is the property of transferring magnetic
induction from one magnet to another by contact or near-contact. The degree of transference
depends upon the magnetomotive force exerted by the modulator, the depth of magnetic
penetration, and subsequent self-demagnetization. Both of the latter functions are
dependent upon the thickness of the carrier, the air gap in the modulator, and the
recorded wave length.
If we could stop the magnetic modulating process instantaneously and examine
the magnetic fields produced within the carrier during the peak energy transfer
period of a weak and strong signal fed into 2 modulators, one with a relatively
large gap and the other with a smaller one, the magnetic field within the carrier
probably would resemble Fig. 10.
Fig. 10-a shows the small degree of penetration obtained with a large gap and
low signal (magnetomotive energy). Fig. 10-b indicates the deeper penetration obtained
with a larger signal. A further increase of signal energy would saturate the carrier.
Fig. 10-c shows the shorter effective magnetic field generated in the carrier by
the small gap. As the gaps become smaller and smaller the induced magnetic fields
become shorter and shorter until the magnetic isolation between pole pieces decreases
to a point where a magnetic short takes place so that no energy is subsequently
available for excitation of the magnetic detector. This is a form of self-demagnetization.
This is roughly equivalent to decreasing the insulation between plates of a condenser
until the leakage becomes so high that virtually no charge remains in the condenser.
Self-discharge in an electrical sense is approximately analogous to self-demagnetization
in a magnetic sense).
Other factors which influence the design and application of magnetic modulators
(recording heads), magnetic detectors (playback heads), magnetic demodulators (obliterating
heads), and their interrelation with the magnetic carrier (wire or ribbon) will
be discussed in a succeeding article Elements of Magnetic Recording scheduled to
appear in the November issue of Radio-Craft.
Comments from readers will be welcomed by the author. Address all correspondence
care of Radio-Craft.
Posted November 11, 2020