Theory
of Operation:
The Quarter Shrinker uses a technique called high-velocity
electromagnetic forming, also known as "Magneforming" or magnetic pulse forming. This is a "high energy rate" process that was originally
developed by the aerospace industry in conjunction with NASA, and was commercialized by Aerovox,
Grumman, and Maxwell Technologies (now part of General Atomics).
High energy rate processes apply a large amount of energy into the
workpiece for a very short interval of time. The approach uses pulsed power
to generate a very high current pulse over a very short time interval.
Our quarter shrinker quickly discharges a high energycapacitor bank through a coil of wire to generate a brief, extremely powerful, rapidly
changing magnetic field which then "forms" (reshapes) the coin. Although electromagnetic forming works best with metals that have
good electrical conductivity (such
as copper, silver, or aluminum), it will also work to a limited extent
with poorer conducting metals or alloys such as steel or
nickel.
In order to
shrink coins, we charge up a high voltage capacitor
bank that consists of two to four large "energy discharge" capacitors. These are specially constructed low-inductance, steel-cased capacitors that can each deliver up to 100,000 amperes (100 kA) at up to 12,000 volts. Each capacitor measures
30" x
14" x 8", weighs 165 pounds, and is designed to have an expected
lifetime of over 300,000 shots at 100 kA/shot. A double-pole double-throw (DPDT) high voltage relay
is used to connect a high voltage AC source through a 40 kV full-wave bridge rectifier
to charge up the capacitor
bank. The relay then disconnects the charged capacitor bank prior to
firing to prevent possible damage to the power supply and
rectifiers.
To shrink a coin, the capacitor bank is quickly discharged through
a single-layer work coil wound from heavy polyimide-amide 200C magnet wire. The coin is centered within the
middle of the coil by a pair
of non-conductive dowels. This orients the coin so that it is in the strongest portion of
the magnetic field. The dowels also prevent the coin from twisting or from being violently ejected from
the coil. The ends of the work coil
are securely bolted to
a pair of heavy copper bus bars. A spark gap is the only affordable switch that can hold off the high voltage and then reliably and efficiently
switch the huge currents used in the coin shrinking process.
We originally used a specially designed three-terminal triggerable spark gap called a "trigatron". The trigatron was "fired" by applying a fast rising high voltage
(~50 kV) pulse to the triggering electrode, which then
caused the main gap
of the trigatron to fire. However, in order to expand the energy range and reduce spark gap maintenance, I converted to a solenoid-driven high-current spark gap that uses 2.5" diameter brass electrodes (similar to those used in the
previous
trigatron switch). When switched, the solenoid drives one electrode close to the
other,
triggering an arc between them. Since the movable electrode does not
quite
contact the fixed electrode, contact welding is avoided. The solenoid switch consistently fires, does not
self-trigger (no unexpected 6 kJ surprises!), and it requires minimal
maintenance.
Once the spark gap fires, current rapidly climbs in
the
work coil at a rate that can approach five billion
amperes/second. As the work coil current increases, it creates a rapidly increasing magnetic field within the work coil.
The natural resonant frequency of the resulting LC circuit (i.e. the capacitor bank
and the inductance of the work coil and wiring) ranges between 8 -10 kilohertz (kHz). Through electromagnetic induction ("transformer action"), a
huge circulating alternating current (AC) is induced within the coin. Because of skin effect, the induced AC current within the coin is confined to thin ring that only penetrates about 1/16 of an inch. Because of Lenz's Law, the magnetic fields of the coin and work coil strongly oppose each
other, resulting in large repulsion forces (called Lorentz forces) between
the work coil and the outer rim of the coin. The opposing magnetic field created by the circulating current ring in the coin prevents most of the
magnetic field from the work coil from penetrating the interior of the
coin. Repulsion forces acting between the coin and the surrounding coil are proportional to
the initial energy stored in
the capacitor bank. Since the stored energy is proportional to the square of the initial capacitor bank
voltage, doubling the bank voltage quadruples the magnetic forces.
We typically store between 2,000 - 9,600 Joules
(watt-seconds) of energy within the capacitor bank. Because this is discharged within 20-40
millionths of a second, the instantaneous power
approaches the
peak electrical power consumed by a large city. The repulsion forces between the
work coil and the coin create radial compressive forces that
easily overcome
the yield strength of the alloys in the coin, causing the coin to shrink in diameter. Applying a 5,000 joule pulse
reduces a US quarter to the diameter of a
dime! During shrinking, radial (outward magnetic pressure) forces cause the work coil to explode
in a potentially lethal spray of copper fragments. Simultaneously,
axial magnetic forces smash the work
coil wires together as the coil is simultaneously expanding in diameter.
In
all cases, the forces acting upon the coil are in a direction that tend
to
increase its inductance.
The coin behaves similar to a short-circuited
single-turn
secondary in a 10:1 step down transformer, and the current induced in
the outer rim of the coin can approach a million amperes.
A US clad quarter is reduced from an initial diameter of 0.955" to
approximately
0.650" in 36 millionths of a second, and the coin's diameter shrinks at a rate of
over 480 miles per hour! In US clad coins, most of
the
induced
current actually flows within the inner copper layer of
the clad sandwich
rather than through the poorer-conducting outer layers. This causes the inner layer to
shrink a bit more than the outermost layers, leading to an "Oreo cookie"
effect on the shrunken coin. The
coin
also becomes thicker as it shrinks in diameter, since the coin's mass,
volume, and density all remain the same before and after shrinking. Oreo cookie and thickening effects can be observed in the following image of a normal and
shrunken US quarter. The slight waviness is a consequence of slight and unavoidable
force imbalances during the shrinking process. This short slide show
(from the Florida State University National High Magnetic Field
Laboratory) provides a good explanation and demonstration of quarter
shrinking. They use #14 AWG wire - we use #10 - #14 AWG magnet wire depending on the size of the coin to be shrunk.
The outer cladding layers of the coin are pulled along for the ride as
the center shrinks - similar to the way continental drift operates in the Earth's crust. This often leads to surface features
colliding. Sometimes, one feature will even flow underneath another
feature! For example, in the following image, notice how some of the lettering on this Delaware
quarter have shifted so that they become partially obscured by various parts of the horse.
Similar effects of intense magnetic forces
are sometimes observed on a much larger scale. Repulsion forces between
primary and secondary windings within large utility power
transformers can
literally tear the windings apart during a high-current short
circuit or rip bus bars from their mounting insulators within electrical substations.
While
the coin is shrinking, similar (but opposite) forces are acting upon the work coil.
Magnetic pressure rapidly expands and stretches the work coil,
causing the insulation to separate
from the wire since the film insulation can't stretch as much as the
ductile copper! The
stretched wire "rapidly disassembles" (explodes!), and fragments of the coil are blown
outward with the force of a small bomb. Small coil fragments
have been measured with velocities of 5,000 fps (>3400 mph, or Mach
4.4). For safety, the work coil must be
housed
inside a blast shield. Our blast shield is made from Lexan
polycarbonate, the same material that's used
to make bulletproof windows. Regions of the blast shield that are in the direct
path of exploding coil fragments are further reinforced with steel
armor plates. Once the work coil disintegrates, any residual energy in the
system is dissipated in a ball of white-hot plasma.
The Quarter Shrinker is designed so that any residual voltage on the
capacitor bank is safely dissipated by a bank of high power wirewound
resistors. The system is triggered from about 15 feet away from
a remote control box. I've found (the hard way!) that 8,000 Joules is
about the maximum energy I can repeatedly use without running a risk of
fracturing the Lexan blast shield from the shock wave. Under the right
conditions,
Lexan does shatter - I've got the pieces to prove it! Other
experimenters
(Rob Stephens, Bill Emery, Phillip Rembold, Ross Overstreet, Brian
Basura, and Ed Wingate) have resorted to using 100% steel enclosures
when running at higher power
levels.
Adding steel plates has stopped our Lexan blast shield from
fracturing. The AR400 steel plates (used on armored Humvees) are well suited to surviving the repetitive bombardment from supersonic coil fragments.
In
2009, the folks at Hackerbot Labs (Seattle, WA) built their own coin
shrinker. By using a special 100,000 frame/second camera, clear Plexiglas
dowels, and carefully pre-triggered electronic flash units, their
partners at Intellectual Ventures, Inc. were able to actually capture
a sequence of images of a quarter AS IT WAS SHRINKING. Because the shrinking process occurred so
rapidly, "shrinking" is only seen during four consecutive frames (or
about 40 millionths of a second).
Our Results:
The largest coin we've shrunk was a Silver Eagle, a silver
coin that starts out being about 1.6" in diameter, and ends up 1.3" in diameter afterward.
At 6,300 joules, a Morgan silver dollar
is reduced from about 1.5" to 1.25" in diameter, and a clad Kennedy
half dollar is reduced to a diameter smaller than a
US Quarter.
At 5,000 joules, quarters will shrink to about the diameter of a dime.
Recent measurements of work coil current during the shrinking process show that the work coil consistently fails just
after the first current peak. Fortunately, virtually all of the coin
shrinkage has occurred by this time. Disintegration
of the coil prevents the energy discharge capacitors from seeing
large voltage reversals that could potentially damage or destroy them. However, the rapid
discharge and extremely high peak currents
are still quite hard on most capacitors. Because of
premature failures with earlier GE pulse capacitors, I've redesigned
the system
to use low inductance Maxwell (now General Atomics Energy Products - GAEP) pulse capacitors that are designed to cope with this abuse. The original GE capacitors began failing
after only 50 - 100 shots. The robust Maxwell capacitors have withstood
well over 8,000 shots with nary a whimper.
Examination of the coil fragments show that the wire has
been substantially stretched (#10 AWG looks like #14 AWG afterward),
it becomes strongly work hardened, and it has periodically "pinched" regions and kinks
caused by the copper being stressed far beyond its yield strength by the
ultrastrong magnetic field. Many fragments are less than 1/4" long, and all
pieces show evidence of tensile fracture at the ends. Since the wire's insulation
is blown off, most fragments are bare copper. The wire often also shows signs of localized melting
on the innermost surface of the solenoid due to "current bunching" from the combination
of skin effect and proximity effect.
The
Quarter Shrinker works very well on clad dimes, quarters, half dollars,
Eisenhower, silver Morgan and Peace Dollars, Susan B. Anthony,
Sacagawea, small Presidential dollars, and many foreign coins.
It works less well with nickel and nickel-copper coins, but
has little effect on plated steel coins. It also works well with
older
bronze and copper-zinc alloy pennies. However, since mid 1982, US
pennies
have been made using a zinc core with a thin copper overcoat. During
shrinking,
the thin copper layer vaporizes and the zinc core melts, leaving an
unrecognizable
disk of molten zinc accompanied by a messy shower of zinc globules
throughout the
blast chamber.
Because of the greater hardness and much poorer electrical conductivity
of nickel-copper alloys, the shrinking process doesn't work as well
with US
nickels, shrinking them by only about 10% even at 6,300 Joules.
A
shrunken coin weighs exactly the
same as a normal size coin. As its diameter shrinks, it becomes
correspondingly thicker, but its volume and density remain constant. Bimetallic foreign coins (with rings and
centers made from different alloys) often show different degrees of
shrinkage based
upon electrical conductivity and hardness of the respective alloys. In some cases, the
center portion shrinks a bit more, loosening or sometimes even freeing it from the outer ring. This occurs with older
Mexican, UK, and French bimetallic, and newer Two Euro bimetallic coins.
Because
of
the extremely high discharge currents
and fast current rise times, capacitors rated for energy discharge applications are fabricated
to
have very low inductance and use special internal construction
techniques to
safely handle the mechanical shock created by magnetic and dielectric
forces
during high current pulse discharges. Unfortunately, our original GE
energy discharge capacitors were simply not constructed for this type of abuse,
and magnetic forces began tearing them apart during every shot. One
unit actually
suffered an internal electrical explosion that ruptured its metal
case, causing it to hemorrhage stinky, arc-blackened capacitor oil and aluminum foil fragments all over the floor.
This was a real hit with the wife! Our newer Maxwell energy discharge
capacitors have proven to be true
"Timex's" - they continue to "take a lickin' and keep on
tickin'".
Can Crushing: A larger diameter 3-turn work coil, operating at lower power
levels, is used to crush aluminum cans. An aluminum soft drink can ends up looking
like an hourglass as the center is shrunk to about half its original diameter.
During can crushing, the coil does not disintegrate due to its more massive design
(#4 AWG solid copper wire) and because the system is fired using a lower energy
level than that used for coin crushing. At higher power levels the can is
ripped apart from the combination of the air inside the can suddenly being
compressed, and heating/softening of the can from the induced currents. Can crushing
also works with steel cans, but the can undergoes greater heating and reduced
shrinkage because of steel's lower conductivity. The "skin depth" in steel is also
much thinner due to its ferromagnetic properties. Since
the work coil is not destroyed during can crushing, the capacitor bank and
spark gap are stressed by a damped oscillatory ("ringing")
discharge.
The capacitor bank voltage must be reduced to so that voltage reversals don't overstress the pulse capacitors.
Since most of the capacitor bank's energy ends up being dissipated as
heat
in the spark gap, can crushing also causes significant heating and erosion of the large electrodes in the
HV switch.
Is Wire Fragmentation Consistent with EM Field Theory?
Copper wire fragments from the work
coil clearly indicate that the wire has been subjected to large tensile stresses.
Most of the observed effects on the wire can be explained by hoop stresses
created by the combination of magnetic pressure
within the work coil solenoid, Lenz's Law repulsion
between the coil and the coin, and periodic conductor necking. The
latter occurs when magnetic
pinch forces are sufficient to cause the conductor to behave as though
it were a conductive fluid. Because of pinch instabilities, the wire becomes periodically pinched off
and broken. However, there is also a
curious ridge which shows
up under microscopic examination of the coil fragments that may hint of
other
effects as well. This artifact was first noticed by Richard Hull of the
Tesla
Coil Builders of Richmond, Virginia (TCBOR) when reviewing similar wire
fragments
from another researcher (Jim Goss). It seems that when an extremely
high
current flows through a solid or liquid metallic conductor, certain
effects
begin to appear which may not be fully explained by existing EM field
theory
and Lorentz forces.
One very interesting example involves forcing a very large
current pulse very quickly through a straight piece of wire. Under
appropriate
conditions, the wire does not melt or explode. Instead, it fractures
into
a series of roughly equal length fragments, with each fragment showing
unmistakable
evidence of tensile failure. Each segment was literally pulled
apart from neighboring fragments with little or no evidence of necking
or melting. Clearly large tensile forces were set up within the wire
during
the brief time that the large current flowed. But, per existing EM
theory,
no tensile forces should exist, implying that the current theory of how
Lorentz forces act on metallic conductors may be incorrect!
A father and son team of physicists, Dr.'s Peter and Neal Graneau (who
are coauthors of "Newtonian Electrodynamics" and "Newton Versus Einstein")
theorize that internally developed "Ampere' tensile forces" may account
for the observed behavior of this, and other high current experiments.
While Ampere' tensile forces are predicted by classical electromagnetic
theory, they have long been removed from all modern textbooks, being replaced
instead by modern field theory and Lorentz forces. Interestingly, even though Ampere' forces
are no longer an accepted part of current EM theory, their existence appears to be experimentally
verifiable in exploding wires or high DC current flow within molten metals (such as aluminum refining).
In their books, the Graneau's provide many thought-provoking
experiments
that appear to support Ampere' Tension forces. More recently, other
scientists have proposed that high current wire fragmentation may actually be
caused by a combination of flexural
vibrations and thermal shock. However, the jury is still out on this issue, and its still an area that's ripe for additional research and experimentation. Isn't Mutilating Money a Federal Offense?
US Federal law specifically forbids
the "fraudulent mutilation, diminution, and falsification of coins" (seeUS
Code, Title 18 - Crimes and Criminal Procedure, Part I - Crimes, Chapter
17 - Coins and Currency, Paragraph 331). However, the key word is Fraudulent.
Although
it recently became illegal to melt pennies or nickels or to export them
to reclaim their value as scrap metal, you can otherwise do pretty much
anything to US coins as long as you don't alter then with an intent
to defraud. This includes squishing
them on railroad tracks, flattening them into elongated souvenirs at
tourist
traps, or crushing them with powerful electromagnetic fields. I
take great pains to
tell folks exactly what they are receiving and how the process was
accomplished.
This is also why those vending machines in tourist traps that squash
pennies
into elongated souvenirs or "funny" stamped pennies with Lincoln
smoking
a cigar are indeed legal (although they can't be used as currency
anymore). Officially, the US Mint "frowns on the despicable practice"
of
altering coins, but they agree that it is quite legal to shrink
coins.
Note that this may not be the case within certain other countries! For example,
in the
UK and Australia, defacing the Queen's image on a coin may still be
considered a punishable offense. Here is an interesting example of fraudulent "coin shrinking" that was prosecuted by the US Secret Service (way back in 1952!).
Paragraph
332
deals with debasement of coins; alteration of official scales,
or embezzlement of metals. Since the coins involved are all made from
base
metals, this section does not apply. However, since the density, metal
content,
and weight remain unaltered during the shrinking process, coin
shrinking
is legal even when applied to coins made from precious metals, and most larger gold and silver
coins shrink quite nicely. HOWEVER, shrinking paper money is
NOT legal. Even though we are aware of a couple of chemical processes that
will shrink dollar bills to about half their original size, we cannot make or
sell "shrunken dollar bills", since defacing paper currency is indeed illegal.
See Paragraph 333 for details.
So Who Invented this Crazy Device?
No, it wasn't me! For the reconstructed history of coin
shrinking, check out The
Known History of "Quarter Shrinking"
“There’s always a hole in theories somewhere, if you look close enough”
Mark Twain, “Tom Sawyer Abroad”
Other References:
Following are various references for the serious researcher.
As many are out of print, you may also wish to check the "Out of Print Books Information" and "In Print Book Sources" sections of the Links Page, or check with your local technical college library system.
A. Electromagnetic Metal Forming and Magneto-Solid Mechanics:
1. ASM, "Metals Handbook, 8th Edition, Volume 4, Forming", American Society for Metals
- see section on Electromagnetic Forming (out of print)
2. Wilson, Frank W., ed., "High Velocity Forming of Metals", ASTME,
Prentice-Hall, 1964, 188 pages (out of print)
3. Bruno, E. J., ed., "High Velocity Forming of Metals", Revised, edition,
ASTME, 1968, 227 pages (out of print)
4. NASA, "High-Velocity Metalworking, a Survey, SP-5062", National Aeronautics
and Space Administration, 1967, 188 pages (out of print)
5. Moon, Francis C., "Magneto-Solid Mechanics", John Wiley & Sons, 1984, ISBN 0471885363, 436 pages (out of print)
6. Murr, L. E., Meyers, M. A., ed., et al, "Metallurgical Applications
of Shock-Wave & High-Strain-Rate Phenomena", Marcel Dekker, 1986,
1136 pages, ISBN 0824776127 (in print) 7.
"Electromagnetic
Forming Handbook" - Currently the BEST Electromagnetic Forming Text, Translated
from Russian and ON LINE.
8. "Pulsed Magnet Crimping" by Fred Niell, straightforward explanation of magnetic forming (fairly technical)
B. Capacitor Discharges, High Magnetic Fields, Pulsed Power/Switching, and Exploding Wires:
1. Frungel, F., "High Speed Pulse Technology", Vol. 3, Academic Press,
1976, 498 pages (Capacitor Discharge Engineering, out of print)
2. Schaefer, Gerhard, "Gas Discharge Closing Switches", Plenum, 1991,
569 pages (out of print)
3. Martin, T. H., et al, "J. C. Martin on Pulsed Power", Plenum, 1996,
546 pages (out of print)
4. Knoepfel, H., "Pulsed High Magnetic Fields; Physical Effects &
Generation…", Elsevier, 1970, 372 pages (out of print)
5. Fowler, C. M., Caird, Erickson, "Megagauss Technology and Pulsed
Power Applications", Plenum; 1987; 879 pages (out of print)
6. Vitkovitsky, Ihor, "High Power Switching", Van Nostrand Reinhold,
1987, 304 pages
(out of print)
7. Pai, S. T, & Zhang, Q., "Introduction to High Power Pulse Technology",
World Scientific, 1995, 307 pages ( in print)
8. Sarjeant, W. J. & Dollinger, Richard E., "High Power Electronics",
Tab Professional & Reference Books, 1989, 392 pages (out of print)
9. Shneerson, G. A., "Fields & Transients in Superhigh Pulse Current
Devices", Nova Science, 1997, 561 pages (out of print)
10. Parkinson, David H., Mulhall, Brian E., "The Generation of High
Magnetic Fields", Plenum, 1967, 165 pages (out of print)
11. Chace, W. G., Moore, H. K, "Exploding Wires", Volume 1, Plenum, 1959, 373 pages) out of print)
12. Chace, W. G., Moore, H. K, "Exploding Wires", Volume 2, Plenum, 1962, 321 pages) out of print)
13. Chace, W. G., Moore, H. K, "Exploding Wires", Volume 3, Plenum, 1964, 410 pages) out of print)
14. Chace, W. G., Moore, H. K, "Exploding Wires", Volume 4, Plenum, 1967, 348 pages) out of print)
15. Mesyats, Gennady A., "Pulsed Power", Springer, 2004, 568 pages, ISBN 0306486531
C. Special Reading for those wishing to delve deeper into more esoteric areas of EM Field Theory and Wire Fragmentation:
1. Graneau, Peter & Neal, "Newtonian Electrodynamics", World Scientific,
1996, 288 pages (in print)
2. Graneau, Peter & Neal, "Newton Versus Einstein, How Matter Interacts
with Matter", Carlton Press, 1993, 219 pages ( in print)
3. Jefimenko, Oleg, "Causality, Electromagnetic Induction, and Gravitation",
Electret Scientific, 1992, 180 pages ( in print)
4. Lukyanov, A., Molokov, S., "Why High Pulsed Currents Shatter Metal Wires?",
Pulsed Power Plasma Science, 2001, Digest of Technical Papers, Volume 2,
pages 1599-1602
5. Lukyanov, A., Molokov, S., Allen, J. E., Wall, D., "The Role of Flexural
Vibrations in the Wire Fragmentation", Pulsed Power 2000, IEE Symposium ,
pages 36/1 -36/4
6. Wall, D. P., Allen, J. E., Molokov, S., "The Fragmentation of Wires
by Pulsed Currents: Beyond the First Fracture", Journal of Physics D: Applied Physics.
36 (2003) 2757–2766
NOTE!
The information on this site is for educational purposes only. It is not to
be construed as advice on how to build or use similar equipment. Electromagnetic
Forming is an extremely dangerous high-energy process that can maim or kill
a casual HV experimenter!
