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All About Quarter Shrinking
(or "Makin' Small Change"©)

Updated 01/27/15

This page is a relatively technical explanation about how our Quarter Shrinker works.
You can also download a simpler one-page PDF summary.


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" 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 energy capacitors 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 "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 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. 

The charged capacitor bank is then quickly discharged into a single-layer 10-turn work coil wound from high temperature (polyimide-amide 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 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 driven spark-gap switch consistently fires, does not self-trigger (i.e., no unexpected high-energy "surprises"!), and it requires minimal maintenance.

Once the spark gap fires, current climbs in the work coil at a rate that can 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 to10 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 wide. And, b
ecause 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 shot size 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 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. 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 coil will always tend to to increase the coil's inductance.

The coin behaves similar to a short-circuited one-turn 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 approximately 0.650" within 36 millionths of a second, and the coin's diameter shrinks at a average rate of over 480 miles per hour! In US clad coins, most of the induced 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 coin 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 (from 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 quarter 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 shrink. 

Coin edge

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 the Delaware quarter below have shifted so that they become partially obscured by various parts of the horse.

Delaware Quarter feature shifting

Similar effects of intense magnetic forces are sometimes seen on a much larger scale. For example, during accidental short circuits, the repulsion forces between primary and secondary windings within large utility power transformers can literally the tear windings apart or rip bus bars from their mounting insulators within large electrical substations.

While the coin is shrinking, similar and opposite forces act upon the work coil. Magnetic pressure rapidly expands and stretches the copper wire in the work coil, 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 Mach 4.4), so the work coil must be housed inside 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 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 walls from the shock wave. Under the right conditions, Lexan does indeed 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 strategically-placed steel plates has stopped our Lexan blast shield from 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. And even these must be periodically replaced after a couple thousand shots.

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 occurs so rapidly, "shrinking" is only seen during four consecutive frames (or about 40 millionths of a second).

Our Results:

The largest coin we've ever shrunk was a US Silver Eagle, a pure silver coin that starts out being about 1.6" in diameter, and ended up ~1.3" in diameter after a 6300 Joule shot. At similar energies, a Morgan silver dollar 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 US Quarter. 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 potentially damage or destroy the energy discharge capacitors. However, combination of high peak currents and oscillatory discharges are still quite demanding for 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. While the original capacitors began failing after only 50 - 100 shots, the more robust 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.

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, and it has very 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. Larger copper-nickel coins, such as the UK Churchill Crown, seem to be almost  impervious to shrinking even at 6300 Joules. The coin seems to be as tough as its namesake!

A shrunken coin weighs exactly the same as a normal size coin. As the coin's diameter shrinks, it becomes correspondingly thicker such that 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. Complete separation occurs with older Mexican, UK, and French bimetallic coins, and with newer Two Euro bimetallic coins.

Because of the extremely high discharge currents and fast current rise times, capacitors rated for energy discharge applications are designed to have high mechanical strength and very low inductance. They use special internal construction techniques to safely handle mechanical stresses created by magnetic and dielectric forces 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 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. 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 tickin'".  

2/4/14 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 circuit, 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 excessively low operating temperatures. These capacitors use a combination of kraft paper and castor oil for the dielectric system that separates the foil plates. Unfortunately, the Quarter Shrinker resides in an unheated patio. Although this has not been a problem during previous Winters, this year was abnormally cold. When the capacitor's internal temperature fell below -10C (14F), the castor oil began to solidify. When castor oil solidifies, its dielectric constant drops from 4.7 to about 2.2, and small amounts of water (that had been harmlessly in solution), were driven out and subsequently absorbed by the kraft paper. The combination caused the internal voltage stress on the kraft paper to increase while the absorbed water simultaneously reduced its electrical strength. The result was 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.

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 damped oscillatory ("ringing") discharge. The 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 heat 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 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, we think that 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" (see US 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 them 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. So vending machines in tourist traps that squash pennies into elongated souvenirs or "funny" stamped pennies with Lincoln smoking a cigar are legal (although the coins can't be used as currency anymore). In an opinion letter, 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 other countries! For example, in the 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!).

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 US 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 do not make or sell "shrunken dollar bills", since defacing paper currency is indeed illegal. See Paragraph 333 for details.

So who Invented this nutty device?

No, it wasn't me! We just perfected the technique. For the recent 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

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. "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)
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

D. Web-Accessible Sources of Information on Ampere' tension forces:

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!  High Voltage

Some Other Places to Go:

Tesla Information Center
Tesla Coil
Link to 345 kV Switch MPEG
Shrunken Coins for Sale Lichtenbergs for Sale
Tesla Info Center
"Quarter Shrinker"
More Arcs & Sparks
Get Shrunken Coins!
Get a Lichtenberg!

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