Build Yourself An Electric Gun

A gun that uses moving electrons instead of messy chemicals to throw a slug has been a staple of speculative fiction since the days of Edison and Tesla—and not only of fiction. Electrically operated projectile launchers—variously known as Gauss rifles, railguns, and mass drivers—have both fascinated and frustrated military researchers the world over [see ”For Love of a Gun,” IEEE Spectrum, July].

So of course I jumped at the chance to build one.

After skimming the Web for sources, I settled instead on a design from the optimistically titled book Mechatronics for the Evil Genius , by Newton C. Braga (McGraw-Hill, 2006), and nipped out to my local electronics shop for some parts.

These shops aren’t what they used to be. There was a drawer for silicon-controlled rectifiers (SCRs)—I’d need one for a fast, high-current switch—but it was empty, and the clerk said there were no plans to restock the item. On top of that, the biggest capacitor on the rack offered a piddling 4700 microfarads. I was lucky to get a transformer, a few packets of resistors within shouting distance of the values I needed, and some wire that just might be suitable for winding a coil.

Even online the pickings were slim: most vendors cater to buyers willing to place bulk orders with plenty of lead time, not writers on deadline. I finally found an outlet that had 22 000-µF capacitors and the SCR I needed and promised to deliver them fast. Then I got the soldering iron and heavy-gauge wire out of the basement and went back to wiring the rest of the circuit and winding my solenoid.

When the capacitors and the SCR arrived, I was eager to get everything hooked together. On the incoming side of the circuit, I had a 12-volt transformer (to make sure I didn’t kill myself), a fairly hefty diode to transform ac into pulsed dc, and a 10-watt, 50-ohm brick of a resistor to limit the charging current for the capacitor so the wires wouldn’t melt. On the output side, I had my coil—160 turns wound around a transparent plastic tube, chosen so I could see the projectile move—and the SCR with a push-button switch controlling voltage to the gate. (You can also substitute a photoresistor for the gate switch, to trigger the SCR automatically.)

I plugged in the transformer, threw the switch to charge the capacitor, waited with bated breath for it to reach maximum voltage, then touched the firing contact.

Tick.

I closed the circuit again.

Tick.

The scrap of metal inside my magnet coil moved perceptibly each time, but that was about it. I guess my concerns about the danger of this home-built electromagnetic cannon were overblown.

It turns out I should have spent a little more time jotting calculations on the back of an envelope. That 22 000-µF capacitor stores a little more than 1/50 of a joule for each volt of potential across it. At the 15 to 20 volts my slapdash circuitry was willing to generate, a perfectly efficient transfer of energy would propel a 25-gram projectile at a blistering 3 meters per second. My toddler can throw harder than that. But I wasn’t getting 3 meters per second. I might not even have gotten 3 centimeters per second.

Back to the envelope. The ideal coilgun uses the interplay between the current-induced magnetic field pulse inside the coil and the movement of the ferrous projectile to maximize the energy transferred from wire to slug. But that requires the slug to zip through the coil in a fraction of a second. My energy transfer was abysmal, as I could tell by the spark when I closed the contact for a second time. Essentially all the energy of the current pulse was winding up right back in the capacitor.

I needed to put more turns in my coils and to compress my magnetic field to a smaller volume. That way, I could get the projectile moving fast enough to play effectively with the emerging magnetic field.

The right approach would have been to find another tube and wind my wire carefully around it or even to get wire better suited for winding solenoid coils. Instead, I just took a fresh spool, unwound a few inches from the outside, and soldered a few inches of heavy-gauge wire onto the nib that projected into the hollow core of the spool. The projectile is smaller, but now when I close the connection it hits the other side of the desk with a satisfying tink.

It’s not going to shoot down an incoming ballistic missile or even seriously annoy our cat, but it’ll do as proof of concept. If I add a second spool and capacitor (or third, or fourth) that can be triggered by a circuit that detects the projectile emerging from the previous one—using, say, a bright light, a photoresistor, and a thin coat of white paint on the slug—I could get some real velocity. I bet I could get the total kinetic energy up to well over a joule.

This little toy also points up many of the reasons that more-powerful Gauss rifles and other electronic projectile throwers still haven’t changed the face of battle. The current through my circuit peaks somewhere around 10 amperes, which is almost 20 times as much as the wire in the coil is rated to carry in continuous duty. A real weapon would be discharging hundreds or even thousands of amperes at hundreds of volts (albeit for only milliseconds at a time) with corresponding stress on capacitors, coils, and switches. That’s fine for a small electric power substation but not much fun to carry over your shoulder.

Still, in the back of my mind I have visions of a Mark II home version. Maybe a huge bank of capacitors scavenged from defunct PC power supplies. Or a bicycle wheel, reinforced and wired into a high-current generator. The underlying idea is so attractive that there has to be a way�.