What coins are magnetic
Theory of Operation:
The Quarter Shrinker uses a technique called high-velocity electromagnetic forming ("EM forming"). This is sometimes called "magneforming" or magnetic pulse forming. It is a "high-energy rate" metal forming process. High-energy rate processes apply a large amount of energy an object for a very short period of time. The technique was originally developed by the aerospace industry in conjunction with NASA, and was then commercialized by Aerovox. Grumman. and Maxwell Technologies (now a subsidiary of General Atomics ). EM forming uses pulsed power technology to quickly discharge high energy capacitors through a coil of wire to generate a brief, but extremely powerful, rapidly-changing magnetic field to re-shape metals in or near the coil. 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 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 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 source through a 40 kV full-wave bridge rectifier to charge up the capacitor bank. After the bank is charged to the desired voltage, the HV relay disconnects the capacitor bank from the charging supply to prevent possible damage to the power supply rectifiers when the system is fired.
The charged capacitor bank is then quickly discharged into a single-layer ten-turn work coil wound from high-temperature (polyamide-imide double-build 200C) magnet wire. The coil has an inner diameter that is slightly larger than the diameter of the coin to be shrunk. The coin is centered and held in the center of the coil by a pair of non-conductive dowel rods. The rods hold the coin in the center of the coil (the strongest portion of the coil's magnetic field) and also prevent the coin from twisting or being ejected from the coil during the shrinking process. The wire 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 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 newer 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 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 combined 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, 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 r epulsion 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 capacitor 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 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 combination of magnetic forces acting upon the work coil will always be in a direction that tend to to increase the coil's inductance.
The coin acts like a short-circuited secondary in a 10:1 step down transformer. The current circulating within 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" 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 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. Despite the radical changes to the coin, its 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 edge-on 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 quarter shrinking. In their demonstration, they use #14 AWG magnet wire for their work coils. We use #10 - #14 AWG wire depending on the size of the coin we're going to shrink.
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 a coin's surface features, and often one feature may sometimes 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.
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 transformers can literally tear the windings apart or rip bus bars from their mounting insulators within 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, and the film insulation peels off the wire since the film 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 the 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 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. But 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).
The largest coin we've ever shrunk was a US Silver Eagle, a pure silver coin that is reduced from 1.6" in diameter to 1.3" 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 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 damage, and eventually destroy, the energy discharge capacitors. The combination of high peak currents and oscillatory discharges is extremely demanding on 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 afterwards), 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 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 - this 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, 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. 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 must be 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 fast, high-current 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 developed an internal short circuit, and all of the stored energy in the capacitor bank (
4.5 kJ) was abruptly 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 had not been a problem during previous winters, 2014 was abnormally cold. When the capacitor's internal temperature fell below -10C (14F), the castor oil began to "cloud" (partially solidify). As castor oil solidifies, its dielectric constant drops from 4.7 to about 2.2. During solidification, small amounts of dissolved water (that had harmlessly been in solution), were driven out of solution and 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 sudden dielectric failure and catastrophic short-circuiting of the capacitor. We've subsequently installed flexible silicone electrical heating elements to the sides of the capacitors to always keep them toasty (above 40F) during even the coldest days. This should prevent any freezing problems in the future. With the new heaters in place, the winter of 2015 proved to be uneventful.
A larger diameter 3-turn work coil, operating at lower power levels, can be 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 lower energy levels than 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 smaller 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 ") discharge. The capacitor bank voltage must be reduced to so that the
100% 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.
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 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" (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 squashing 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, folks at the US Mint "frown on the despicable practice" of altering coins, but they agree that it is quite legal to shrink coins. Note that this is not always 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 most of the coins we shrink are 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 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.
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"