Category Archives: physics

The Quarter Shrinker returns

Back in 2009, I was one of several folks that built the coin shrinker at Hackerbot Labs.  After several hundred firings and sitting in a closet for months, it badly needed some love. We moved it to my shop and I got to work.

As with most projects on this site, the hazards here are many and subtle and include high voltage, extreme UV production, supersonic shrapnel, a several Tesla EMP, charging and discharging hazards, toxic smoke… In short, DON’T TRY THIS AT HOME. Come to my shop instead and we’ll shrink some coins. ^_^

Here’s a breakdown of how the Quarter Shrinker does its magic.

This machine eats kilojoules, copper, and vice-grips to make tiny coins.

Energy is stored in three 10kV/100uF Aerovox capacitors wired in parallel with thick copper bus bar. The capacitor bank is quite heavy, weighing in at about 200 kg (450 pounds). When fully charged, the capacitor holds 15,000 joules. Is this dangerous? To quote Wikipedia,

Any capacitor containing over 10 joules of energy is generally considered hazardous, while 50 joules or higher is potentially lethal.

Caution is clearly advised.

(Aside: we do in fact see dielectric absorption hysteresis after every firing. This toy can be lethal even after you think you’ve turned it off!)

Copper bus bars. For when the current absolutely, positively has to be there on time. The jumper cable safety short keeps the capacitor “turned off” when not in use.

The capacitor is connected to a high voltage DC power supply for charging. It includes a neon sign transformer, variac, and high voltage rectifier. The state of charge of the capacitor is monitored using a cheap semi-disposable volt meter and a 1000:1 voltage divider. Every volt on the meter represents 1000V on the bank.

There’s a gigohm resistor between the red lead and the positive terminal of the capacitor. The internal resistance of the meter is 1 megohm, making a convenient voltage divider of 1000:1.

The red and black twisted leads are connected to a hinged plate that is raised during use, but can be brought down at any time by pulling a long rope. Doing that immediately puts a large resistor across the capacitor, quickly and safely discharging it.

Resistance is not futile. In fact, it’s absolutely necessary for safely discharging the cap bank.

The main switch is a mechanical trigger with tungsten carbide contacts and an HDPE and delrin housing. It too can be gravity-closed by pulling a long rope. This is assisted by a rubber band made of surgical tubing, closing the switch as quickly as possible.

The trigger is shaped so that the contacts come very close together, but don’t actually touch. This keeps them from welding to each other or getting damaged from physical impact.

The trigger. Aim away from face when firing.

Current starts to flow well before the contacts come together, creating a fantastically bright flash of light (including ridiculous amounts of UV). Do not look at trigger with remaining eye.

The light on the left is the trigger closing. The light on top is the coil exploding.
The light on the left is the trigger closing. The light on top is the coil exploding.

The big plastic box on top is the blast chamber. It’s designed to absorb the force of the exploding coil without  breaking. A baffle system allows the rapidly expanding air and copper vapor to escape while trapping the copper shrapnel inside.

There’s a quarter inside that coil, about to be shrunk.

The copper coil consists of twelve windings of 12 gauge solid copper wire. Vice grips clamp the wire to the copper bus bar. As with nearly everything on this machine, the vice grips, sacrificial chunks of HDPE, and bus bar ends are all semi-disposable.

Physics Girl has a fantastic description of the physical forces at play, including how the quarter shrinks and why the coil explodes.

After pulling the trigger, the capacitor discharges in to the coil, shrinking the coin in less than 40 microseconds. The explosion is extremely loud. Ear protection, distance from the machine, and yelling “fire in the hole” are all mandatory.

The resulting shrunken coin glows white-hot during the process, and is quite hot for several minutes afterwards. Some of the copper coil is vaporized by the electric arc, creating a green copper plasma. The coin is typically covered in an atomically thin layer of copper, discoloring it. (Note to self: try to get an SEM photo of the copper plating on a coin when Millie is back online.)

Today’s money just doesn’t go as far as it used to. Also, copper plating a couple of atoms thick.

My shop-mate Sirus made a nice 4k video of the setup process and first post-rebuild firing. (The charge time is probably greater than your attention span. To skip directly to the trigger pull, CLICK HERE.)

SEM FTW: Meryl lives again

Happy Holidays!

Here is a 2mm stainless steel screw. It’s sitting on a standard, non-shrunken quarter for scale.

This screw is just 2mm across, and is about as long as the LIBERTY on a quarter.
This screw is just 2mm across, and is about as long as the LIBERTY on a quarter.

Here it is under Meryl’s beam at the lowest possible magnification.

2mm screw at 35x. The bar scale at the bottom shows a reference line 0.5 mm long.
2mm screw at 35x. The bar scale at the bottom shows a reference line 0.5 mm long.

Those threads are barely visible to the naked eye, but even at low magnification the stainless shows signs of galling.

Let’s enhance.

The tip of each thread is just over .06mm, but the edges have obvious surface variation.
The tip of each thread is just over .06mm, but the edges have obvious surface variation.

At this magnification, the tip of the thread looks a bit like lumpy clay.

Can we get closer? Of course.

At 1100x, a single thread on this tiny screw looks like a vast chasm.
At 1100x, a single thread on this tiny screw looks like a vast chasm.

Steel is a decent electrical conductor, so it’s relatively easy to image. Insulators (like most organic matter) are a lot tougher to shoot without special processing (such as sputter coating).

Here is a seashell:

I bet this little creature never expected to be shot with an electron beam. That's life, I guess.
I bet this little creature never expected to be shot with an electron beam. That’s life, I guess.

Here is the same shell at low magnification:

At low magnification, the shell resembles a diabolical, Giger-esque landscape.
At low magnification, the shell resembles a diabolical, Giger-esque landscape.

See the jets of “water” shooting off of the spikes to the left? That’s not photoshop, that’s physics.

As the electron beam scans the surface, the shell (being a poor conductor) accumulates charge. Over time the shell becomes more negatively charged. Like charges repel, so the electron beam is deflected.

The field in the shell will be strongest in the places with the strongest curvature (the tips of those spines). Since we’re using the beam to make the image itself, that’s exactly where the image will be the most distorted.

Apparently this shell has the ability to warp spacetime. Maybe I'd better back off...
Apparently this shell has the ability to warp spacetime. Maybe I’d better back off…

At higher magnification the problem only becomes worse.

I’m glad Meryl is finally taking decent (if not quite breathtaking) photos. If you could look at whatever you wanted under an SEM, what would it be?

Next project: a DIY sputter coater!

More SEM photos

A tale of two microscopes

I recently became the proud owner of a couple of discarded JEOL scanning electron microscopes. The big one (“Milly“) is a JSM-6320F from the early 1990s. The little one (“Meryl“) is a JSM-5600 from the early 2000s.

Milly the SEM.
This is Milly. She’s large and in charge.
This is Meryl. She may be small, but I wouldn't cross her.
This is Meryl. She may be small, but don’t let that fool you. She’s plenty of trouble.

They sat in storage for about five years, and the previous owner had a lot of trouble getting them running again. That trouble has now passed on to me, and I’m in the process of restoring them to their former glory.

Where to begin

Of the two, Milly is a lot more technically interesting. She uses an FEG emitter (requiring an ultra-high vacuum to operate). She has an SEI imager and an X-ray backscatter detector, and was once capable of producing extremely impressive images at about 1nm resolution.

On the downside, she’s a little complicated. Her vacuum system uses two roughing pumps (not supplied), two diffusion pumps, and three ion pumps. Getting the vacuum down below 10-9 Torr requires perfect seals and a finicky bake-out process. Her power requirements are a little fancy. And being late 1990s technology, her “computer” looks like something used on one of the Apollo missions.

Old console is old.
This console features not two, but three CRTs (counting the Polaroid film scanner tube).

In addition to these challenges, I’ve never actually used (let alone worked on) an SEM. While Milly might eventually take stunning images, I have a feeling that the road to getting there may be a long one.

Lucky for me, Milly has a little sister.

And then there’s Meryl

Meryl is a much simpler SEM. She uses a thermionic emitter (a simple tungsten filament) rather than an FEG. She doesn’t require UHV, so her seals are simply rubber gaskets. There is only one roughing pump (provided!) and a single diffusion pump. She only needs 100V AC, which is easily converted from standard 110 with a supplied variac. She only takes up about half the space of her big sister. Best of all, she even came with a few spare parts, which considering my inexperience, I fully expect to install.

Oscilloscope says... not enough vacuum.
Does the patient have a pulse? Pirani gauge says… not yet.

Watch this space for updates as the great microscope adventure unfolds.

Bussard on polywell fusion

Here is the classic Google Tech Talk from Robert Bussard (yes, the ramjet guy). In it he talks about a design for a novel fusion reactor he developed for the DoD over the course of eleven years. He was hoping to find further funding for the project in the Silicon Valley tech set. Sadly, he died less than a year after this talk, but interest in the design continues.

Folks like Famulus are trying to recreate Bussard’s work in an attempt to create a DIY-able fusion reactor source. Is it possible that the next energy revolution will come from a garage inventor?