64 PREMIER GUITAR NOVEMBER 2009 www.premierguitar.com
Alnico Magnets Today
Alnico permanent magnets are made from
an alloy containing varying percentages of
iron, aluminum, nickel, cobalt, copper and
sometimes titanium. It is produced in different grades, which offer increased magnetic
strength (maximum energy product) and resistance to demagnetizing forces (coercivity).
Alnico offers the best temperature stability of
any standard magnet material produced, but it
is also the most susceptible to demagnetizing
forces. It is often selected for modern products
that must operate at extremely high temperatures. In guitar speakers, alnico is usually
selected to help recreate the sound of ‘50s and
‘60s blues, jazz and rock and roll music.
Electrodynamic Loudspeaker History
One of the first modern-style (
electrodynamic or “moving coil”) loudspeakers, the
Magnavox, was demonstrated in 1915 by
Edwin S. Pridham and Peter L. Jensen. Their
1920 patent application (US Patent 1448279)
describes “...an annular coil rigidly connected
to a diaphragm. This coil is disposed, so as
to be freely movable, in a strong concentric
magnetic field produced either by a permanent or an electromagnet.” Today, the vast
majority of speakers use permanent magnets;
however, in the 1920s the permanent magnets available were relatively weak, and those
capable of producing a strong magnetic force
were costly and difficult to make. Most early
electrodynamic loudspeakers used an electromagnet (also known as the field coil).
Electromagnet vs. Permanent Magnet
An electromagnet is composed of a coil
wrapped around an iron core. When DC current
passes through the coil it generates a steady
magnetic field. Electromagnets may be thought
of as temporary magnets that lose their magnetism when power is shut off, while permanent
magnets do not require an electrical power
source to remain magnetized.
Alnico Magnet Development
In the early 1930s, most of the world was in the
depths of the economic downturn known in
the US as the Great Depression. Early patents
relating to the development of alnico express
their objective to provide a permanent magnet
composed of relatively low-cost materials,
which could be fabricated inexpensively and
have superior magnetic characteristics.
In US Patent 2027994 (Applied For:
1/20/1932; In Japan 3/09/1931), inventor
Tokushichi Mishima of Japan explains how
a strong permanent magnet comprised of
nickel, aluminum and iron could be produced
economically and with superior magnetic characteristics to the “magnet steels” available
at the time, including tungsten, chrome, and
chrome manganese. Months later, he added to
this the discovery that the addition of cobalt
could further improve the magnetic characteristics of the alloy, as well as improving its
tenacity and ductility (US Patent 2027996).
In the Nov. 4, 1935 issue of Time magazine,
an article titled “Science: Industrial Insides”
describes researcher William E. Ruder of GE
Schenectady demonstrating the power of the
new alnico permanent magnet by swinging a
55-lb. radio cabinet from an alnico disk of less
than a pound. “Alnico is being groomed to
displace small electromagnets in motors, transformers, and loudspeakers, lowering the cost
and simplifying the construction.”
In US Patent 2295082 (Applied For:
6/29/1939; In Germany 12/06/1938), inventor Gottfried Bruno Jonas of the Netherlands
explains that an alnico alloy in the anisotropic
form can yield a permanent magnet with a
50% to 200% higher maximum energy product than the isotropic version. This is the
discovery behind what would become known
as Alnico 5. This application was patented on
Sept. 8, 1942 in the United States.
Alnico had been developed into a robust permanent magnet by 1939. In that same year,
Charlie Christian joined Benny Goodman’s
band and is credited with popularizing the
electric guitar as a lead solo instrument on
par with the trumpet and saxophone, but
his Gibson EH- 150 amplifier used a field-coil
speaker. It was not until about 1947 when
Gibson and Fender began stocking their
guitar amps with alnico speakers. What could
account for such a delay?
Don’t you know there’s a war on?
The Nazis invaded Poland in 1939 and WWII
began in Europe. In January of 1942, a month
after entering the war, the United States
government established the War Production
Board to regulate the production and allocation of raw materials. Basic metals including
steel, copper, and aluminum were heavily
regulated to supply military demand, and
only essential civilian products would be allocated a percentage of these metals. In order
to stay profitable, many American companies
had to shift their production to supply parts
and equipment the military needed. For
example, Gibson made radar assemblies,
glider skids for airplanes and precision rods
for use in submachine guns. It would not be
until after the war ended in 1945 that many
peacetime products could make full use of
alnico, the miracle metal.
KURT PRANGE
SPEAKER TONE CENTER
Alnico, the Miracle Metal
Kurt Prange
Kurt Prange (BSEE) is the Sales Engineer for CE Distribution,
LLC, in Tempe, AZ ( jensentone.com). Kurt began playing guitar at the age of nine in Kalamazoo, MI. He is a guitar DIY’er
and tube amp designer who enjoys helping other musicians
along in the endless pursuit of tone.
An advertisement from QST amateur radio magazine
(April 1945, p. 75)
