A conventional cannon has some type of shell in a tube. The shell is then launched by the expansion of exploding gun powder. What about a railgun? This weapon can fire a projectile at tremendous speeds without even using an expanding gas. But how does it work?

Let's go over some of the basic principles for the railgun to operate.

Electric Currents Make Magnetic Fields ————————————–

Here is a simple experiment. You probably have the materials so that you can try this at home. Well, you might not have a magnetic compass - but you should get one anyway (your phone won't work in a zombie apocalypse). Next take a wire (any conducting wire should probably work) and place the wire such that it is oriented along a North-South line over the compass. Like this:

When you connect this wire to a battery, you will see the magnetic compass needle below the wire move a little bit. How much it moves depends on how much electric current is running through the wire (and how close the needle is to the wire). Don't keep the wire on the battery for too long, it will get hot.

This simple compass demos shows something very important. Electric currents create magnetic fields. These magnetic fields interact with the compass and make it move.

The pattern of this magnetic field is important also. If you were to look at the direction of the magnetic field a different locations around the wire, it would look like this:

Screen capture from a VPython program showing the calculation of the magnetic field.

I have decided that it's easier to just actually calculate the magnetic field and display the vectors with VPython than to just draw a sketch.

Magnetic Fields Push on Electric Currents —————————————–

If something makes a magnetic field, then that same thing will experience a force when placed in an external field. Here is another quick demo.

The wire with current is near the magnet. When current runs through the wire, there is a magnetic force on the wire which causes it to swing. What about the direction of the force? This force is perpendicular to both the direction of the current and the direction of the magnetic field. In this example (from the video), the current is to the left and the magnetic field is upwards. There are only two directions perpendicular to both of these vectors. One direction is in the direction the wire swings.

The Railgun ———–

Putting these two ideas together, you can make a railgun. The device is simple in design. You have two parallel rails (thus called a railgun) and a moveable projectile that is also like a wire. An electric current goes down one wire, across the projectile and then back down the other rail. In between the two parallel rails, both of the magnetic fields due to the rails points in the same direction and make a stronger magnetic field. This magnetic field then pushes on the projectile with the current running through it to propel it out of the railgun. Boom. A projectile.

Maybe this diagram will help visualize what's going on.

Screen capture of a VPython program showing the force on a wire.

The cyan arrows represent the magnetic fields from the two rails. The red arrows are electric current and the gray arrow is the force vector on the moveable wire crossing the two rails. That's your railgun.

Could You Build a Railgun? ————————–

The idea of a railgun isn't so difficult. It seems like I could build one that wouldn't shoot very fast, but at least it could demonstrate the idea. Maybe my moveable wire just moves a little bit instead of blasting out like a cannon - that would be fine with me.

Here is the demonstration railgun I started with.

Image: Rhett Allain

The two thicker rails are held into a parallel position with some Lego pieces and connected to a power supply. Across the rails is a thin wire to act as the "projectile". I actually started with a metal ball on the rails. I figured the ball would roll better and look cooler.

I was wrong. This didn't work. I even turned the power supply up so that there was 10 amps of electric current running through the rails. Nothing happened.

Ok. How much current would I need to get this thing to work? Or maybe a better question: what kind of force would be on the wire with a 10 amp current?

Here is the perfect place for a back-of-the-envelope calculation. The idea is to make some basic assumptions to get a rough estimate for the value of the force on the wire. It doesn't have to be a perfect estimation, just within an order of magnitude would be fine.

Here are my assumptions:

The magnetic field between the two rails has a constant value. Of course this is wrong, but I don't care.

The magnetic field in the center of the two rails can be calculated using the formula for the magnetic field due to a long wire. Again, this is wrong. The "long wire" formula assumes you are in the middle of a long wire. In this case, there is no electric current in the rail past the cross over wire.

Now for the calculation. The magnetic field due to a long straight wire would be:

The μ-4~0 over 4π is just a constant. r is the distance from the center of the wire. If I use a distance of 2 cm and a current of 10 amps, I get a magnetic field of 2 x 10-4 Tesla.~

Now if I have the same 10 amp current running through the 2 cm long cross over wire (projectile), I can use the following to calculate the force on a wire with current:

Since the magnetic field and the current are perpendicular, this is easy to calculate the magnitude of the force. I get a value of 4 x 10-5 Newtons.

That's not a very large force. What if I increased the current? Actually, since both the force and the magnetic field are proportional to the current, doubling the current would increase the force by a factor of 4. Ok, let's say I had 100 amps in my rail. This would increase the force to just 4 x 10-3 N. That's still not enough. Also, there is no way my power supply could get up to 100 Amps.

What about 1000 amps? Yes, that might do it. Really, the only way to get that high of a current is with some type of capacitor bank that can be discharged very quickly. But wait! If I have 1000 amps in the rails, won't the rails also push on each other? Yes.

Like I said, I'm not going to build a demonstration version of a railgun.

Build your own railgun.

Yes, you actually can build a railgun - but it's dangerous. This site has some instructions how how to do it. Notice that the first railgun they build uses magnets. This is a simple demonstration, but it is not actually a railgun. The railgun doesn't use permanent magnets.

Maybe I should also point out that there is a difference between a railgun and a coilgun. A coilgun uses a series of electromagnetic coils to accelerate a ferromagnetic projectile. For the railgun, the projectile is accelerated because of a current running through the projectile. This means that it only needs to be an electric conductor and not a ferromagnetic material.