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" metal forming process that was originally
developed by the aerospace industry in conjunction with NASA, and was commercialized by Aerovox,
Grumman, and Maxwell Technologies (now a subsidiary of General Atomics).
High energy rate processes apply a large amount of energy into a
workpiece during a very short interval of time. The approach uses pulsed power, quickly discharging high energycapacitors through a coil of wire to generate a brief, but extremely powerful, rapidly-changing magnetic field which 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 nickel or steel.
In order to
shrink coins, we charge up a high voltage capacitor
bank consisting of two to four large capacitors. These "energy discharge" capacitors 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
14" x 8", weighs 177 pounds, and is designed to have an expected
lifetime of over 300,000 shots at 100,000 amperes/shot. A double-pole double-throw (DPDT) high voltage relay
is used to connect a variable high voltage AC power supply through a 40 kV full-wave bridge rectifier
to charge up the bank of capacitors. Once the bank is charged to the
desired voltage, the relay disconnects the capacitor bank from the
charging supply to prevent possible damage to the HV rectifiers when the system is fired.
charged capacitor bank is then quickly discharged into
a single-layer ten-turn work coil wound from high temperature
(polyimide-amide double-build 200C) magnet wire. The coin is centered and held in the
center of the coil by a pair
of non-conductive dowels. This positions the coin so that it is in the
strongest portion of
the coil's magnetic field. The dowels also prevent the coin from twisting or
from being ejected from
the coil during the shrinking process. 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.
For many years, we 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 a trigger electrode, which then
caused the main gap
of the trigatron to fire. However, in order to broaden the operating
voltage range and reduce spark gap maintenance, we converted to a solenoid-driven high-current spark gap that uses 2.5" diameter brass electrodes (similar to those used in the
trigatron switch). When switched, the solenoid drives one electrode close to the
triggering an arc between them. Since the movable electrode does not
contact the fixed electrode, contact welding is avoided. The newer solenoid-driven spark-gap switch consistently fires, does not
self-trigger (i.e., no unexpected high-energy "surprises"!), and it requires minimal
Once the spark gap fires, current climbs in
work coil at a rate that may approach five billion
amperes per 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 (the capacitor bank
and the inductance of the work coil, cables and bus bars) ranges between 7.8 to 10 kilohertz (kHz). Through electromagnetic induction ("transformer action"), a
huge circulating alternating current (AC) is induced within the coin. Due to skin effect,
the induced current within the coin is confined to the outermost rim of
the coin, forming a ring of current only about 1/20 of an inch thick. And, because of Lenz's Law, the magnetic fields from the coin and work coil strongly oppose each
other, resulting in tremendous repulsion (called Lorentz forces) between
the work coil and the outer rim of the coin. The repulsion forces acting between the rim of 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 voltage quadruples the magnetic forces.
We typically use a pulse of 2,000 to 9,600 joules
(watt-seconds) from the capacitor bank. Because this energy is discharged within 20-40
millionths of a second, the instantaneous power
peak electrical power consumed by a large city. The repulsion forces between the
work coil and the coin create radial compressive forces that
the yield strength of the alloys in the coin, causing the coin to shrink in diameter. A 5,000 joule pulse
will reduce a US quarter to the diameter of a
dime. Simultaneously, powerful outward forces ("magnetic pressure") causes the work coil to explode
in a potentially lethal shower of copper shrapnel. Axial magnetic forces also smash the work
coil wires together as the coil is simultaneously expanding in diameter.
The combined forces acting upon the work coil will always tend to to increase the coil's inductance, i.e., squeezing it axially and expanding it radially.
The coin behaves like a short-circuited secondary in a 10:1 step down transformer. The current circulating within
the outer rim of the coin may approach a million amperes!
A US clad quarter is reduced from an initial diameter of 0.955" to
0.650" within 36 millionths of a second. The coin's diameter shrinks at a average rate of
over 480 miles per hour! In US clad coins, most of
current actually flows within the pure copper layer of
the clad sandwich
rather than through the poorer-conducting outer layers. This causes the copper center layer to
shrink a bit more than the outermost layers, leading to an "Oreo cookie"
effect on the shrunken coin. The
also becomes thicker as it shrinks in diameter, and the coin's mass,
volume, and density all remain the same before and after shrinking. The Oreo
cookie and thickening effects can be easily seen in the following image
of a normal-size and
shrunken US quarter. The slight waviness in the shrunken coin is a
consequence of unavoidable
force imbalances due to thickness variations (i.e., the coin's surface features) and slight coil asymmetries. This short slide show from the Florida State University National High Magnetic Field
Laboratory provides an excellent explanation and demonstration of
shrinking. In their demonstration, they use #14 AWG magnet wire for their coils. We use
#10 - #14 AWG wire depending on the size of the coin we're going to
In clad coins, as the copper core layer shrinks, the outer cladding
layers of the coin are pulled along for the ride, similar to the way
continental drift moves continents in
the Earth's crust. This often leads to "collisions" between surface
features, and often one feature may plow underneath another!
For example, note how some of the lettering on
quarter below have shifted so that they become partially obscured by various
parts of the horse.
Similar effects of intense magnetic forces
are sometimes seen on a much larger scale: During accidental short circuits, the repulsion forces between
primary and secondary windings within large utility power
literally the tear windings apart or rip bus bars from their mounting insulators within electrical substations.
coin is shrinking, similar and opposite forces act upon the work coil.
Magnetic pressure rapidly expands and stretches the copper wire in the work
causing the insulation to peel off the wire since the film insulation can't stretch as
much as the copper! The 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 up to 5,000 fps (>3400 mph, or greater than Mach
4), so the work coil must be
contained within a heavy 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 10 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 walls from the shock wave. When slammed by a
high-intensity shock wave, Lexan does indeed shatter - I've got the
pieces to prove it! Other
(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
Adding strategically-placed steel plates has stopped our Lexan blast
fracturing. We've found that AR400 steel plates (also used for armor in Humvees!)
are well suited to surviving repetitive bombardment from supersonic
coil fragments. But even these must be periodically replaced after a couple
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 occurs so
rapidly, "shrinking" is only seen during four consecutive frames (or
about 40 millionths of a second).
The largest coin we've ever shrunk was a US Silver Eagle,
a pure silver
coin that started at 1.6" in diameter, and ended up ~1.3" in
diameter after a 6300 Joule shot. At similar energies, a Morgan
is reduced from about 1.5" to 1.25" in diameter, and a clad Kennedy
half dollar is reduced to about the diameter of a
At 5,000 joules, US clad quarters shrink to about the diameter of a dime.
A few years ago, physicist Dr. Tim Koeth and I took various measurements
of work coil current during the shrinking process. These showed that
the work coil consistently failed shortly after the first current peak.
Fortunately, virtually all of the coin's
shrinkage has occurred by this time. Disintegration
of the coil helps to reduce the voltage reversals that could damage, and eventually destroy, the energy discharge capacitors.
However, the combination of high peak currents and oscillatory discharges
are still quite demanding for most capacitors. Because of
premature failures with earlier GE pulse capacitors, the current system uses low inductance Maxwell (now General Atomics Energy Products - GAEP) pulse capacitors that are designed to safely cope with this abuse. While the original GE capacitors began failing
after only 50 - 100 shots, the trusty Maxwell capacitors have withstood
well over 6,000 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.
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, and
it has little effect on plated steel coins. It also works well with
bronze and copper-zinc alloy pennies. However, since mid-1982, US
have been made using a zinc core with a thin copper overcoat. During
the thin copper layer vaporizes and the zinc core melts, leaving an
disk of molten zinc accompanied by a messy shower of zinc globules
Because of the greater hardness and much poorer electrical conductivity
of nickel-copper alloys, the shrinking process doesn't work as well
nickels, shrinking them by only about 10% even at 6,300 Joules. Larger
copper-nickel coins, such as the UK Churchill Crown, seem to be almost
impervious to shrinking even at 6300 Joules - this coin seems to be as tough as its namesake!
shrunken coin weighs exactly the
same as a normal size coin. As the coin's 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
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. Complete separation occurs with older
Mexican, UK, and French bimetallic coins, and with newer Two Euro bimetallic coins.
the extremely high discharge currents
and fast current rise times, capacitors rated for energy discharge applications are designed
have high mechanical strength and very low inductance. They use special internal construction
safely handle mechanical stresses created by magnetic and dielectric
during rapid, high-current pulse discharges. Unfortunately, the 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
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.
The wife was not amused! Our Maxwell energy discharge
capacitors have proven to be true
"Timex's" of the pulsed power world - they continue to "take a lickin' and keep on
Update - One of our Maxwell capacitors finally failed. While charging
the bank, a muffled bang was heard, the bank voltage
abruptly plunged from about 8 kV to zero, and the mains fuse in the
power controller blew. The problem was traced to one of the Maxwell
capacitors. The failing capacitor had developed an internal short
and all of the stored energy in the capacitor bank (~4.5 kJ) was
suddenly dumped into the internal fault. Fortunately, the heavy steel
case didn't rupture, so I was spared cleaning up several gallons
of castor oil. This capacitor and an identical mate had survived over
6,000 "shots" in the quarter shrinker, so I'm very satisfied with its
performance. Further research determined that the root cause of the
failure was not lifetime-related, but was due to an extended period of abnormally low temperatures. These capacitors use a combination of kraft paper and
castor oil for the dielectric system that separates the foil plates. The Quarter Shrinker resides in an unheated patio.
Although cold temperatures were not a problem during previous Winters, 2014
was abnormally cold. When
the capacitor's internal temperature fell below -10C
(14F), the castor oil began to solidify. When castor oil solidifies, its
constant drops from 4.7 to about 2.2. Also, a small amount of water (that
had previously been harmlessly in solution), was driven out of the
solidifying oil and then absorbed by the kraft paper. The reduced
dielectric constant increased the voltage
stress on the kraft paper dielectric while the absorbed water
simultaneously reduced its electrical strength. The result was a sudden
dielectric failure and catastrophic destruction of the capacitor. We
have now installed flexible silicone electrical
heating elements to the sides of the capacitors to always keep them
above 40F - this should prevent any freezing problems in the
future. With the heaters, the Winter of 2015 proved to be uneventful.
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 electrical 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 more heavily stressed by the oscillatory ("ringing")
capacitor bank voltage must be reduced to so that voltage reversals
don't overstress the pulse capacitors' dielectric system.
Since most of the capacitor bank's initial energy ends up being dissipated as
in the spark gap, can crushing also causes significant heating and
erosion of the electrodes in the high voltage 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
effects as well. This artifact was first noticed by Richard Hull of the
Coil Builders of Richmond, Virginia (TCBOR) when reviewing similar wire
from another researcher (Jim Goss). It seems that when an extremely
current flows through a solid or liquid metallic conductor, certain
begin to appear which may not be fully explained by existing EM field
and Lorentz forces.
One very interesting example involves forcing a very large
current pulse very quickly through a straight piece of wire. Under
conditions, the wire does not melt or explode. Instead, it fractures
a series of roughly equal length fragments, with each fragment showing
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
the brief time that the large current flowed. But, per existing EM
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
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, we suspect that the jury is
still out on this issue, and its 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.
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 them with an intent
to defraud. This includes squashing
them on railroad tracks, flattening them into elongated souvenirs at
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
So vending machines in tourist traps that squash
into elongated souvenirs or "funny" stamped pennies with Lincoln
a cigar are legal (although the coins can't be used as currency
anymore). In an opinion letter, folks at the US Mint "frown on the despicable practice"
altering coins, but they agree that it is quite legal to shrink
Note that this is not always the case within other countries! For example,
UK and Australia, defacing the Queen's image on a coin may 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!).
deals with debasement of coins; alteration of official scales,
or embezzlement of metals. Since most of the coins we shrink are made from
metals, this section does not apply. However, since the density, metal
and weight remain unaltered during the shrinking process, coin
is legal even when applied to bullion coins made from precious metals, and most larger gold and silver
coins shrink quite nicely. HOWEVER, shrinking US paper money is
NOT legal. Even though we are aware of a couple of chemical processes that
can shrink dollar bills to about half their original size, we do not make or
sell "shrunken dollar bills", since defacing paper currency is indeed illegal.
See Paragraph 333 for details.
Are you the nut who invented this device?
No, it wasn't this nut! We just perfected the technique. For the 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”, Charles L. Webster & Co., 1894
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. "Pulsed Magnet Crimping" by Fred Niell, straightforward explanation of magnetic forming (fairly technical, written by a physicist)
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 some "interesting" 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,
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
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.
forming is an extremely dangerous high-energy process that can maim or
a casual HV experimenter. Large high-voltage capacitors are VERY
unforgiving, and they will NOT give you a second chance!