PERMANENT MAGNET GUNS

How to make them and how they work

Is this the next great techno-weapon?

While permanent magnet guns are fascinating, I have to warn visitors that they are never going to represent a threat to world peace. Still, as a science project they are well worth the minor expense and time it takes to build them. Here's how to build one and how it works:

Permanent magnet guns, or PMGs, consist of pairs of ceramic permanent magnets mounted in a "V" configuration with a channel between them through which a magnetic projectile can slide.

The rectangular ceramic magnets are magnetized so that the poles are on the faces of the bar, as the following image shows:

In 2013 twelve could be purchased from Amazon.com for $7 or fifty for $25. They measure 1 and 7/8 x 7/8 x 3/8 inches and can also be found in teacher supply and science supply stores.

These magnets are mounted on any flat surface, usually a piece of wood eight inches wide and four feet long, in a "V" configuration 45-degrees from the perpendicular. In the example above, the angle is set by two pieces of 1 x 2-inch wood (pieces A and B) cut at 45 degrees. The magnets don't like being angled so they need to be firmly secured. A layer of 2-inch wide masking tape across their tops helps lock them in place. It isn't shown in the above image so the orientation of the magnets can be seen more clearly. The angle blocks are glued to the base board as well as two 3/4 x 3/4 inch strips of wood (C) 18 inches long. The strips are 9/16 inch apart (D) and serve as the bore of the permanent magnet gun. Be careful when gluing them to the base so that no glue is pushed into the bore. If this happens, it can prevent the projectile from sliding freely. Double faced indoor/outdoor carpet tape (F) on the base and the faces of the angle blocks (A and B) hold the magnets in place. A strip of 1/2 x 3/16 balsa wood is laid down on the bottom of the bore using double faced tape. This lifts the projectile so that it moves just below the center of the magnetic fields rather than the bottom where it curves and is weaker. A thicker strip that places the projectile at the very center of the angled magnets may result in the projectile flipping out of the bore. The reason is that field curvature in the angled magnets cause the end of the projectile to become slightly lifted. When the projectile is accelerated, this angle causes the lower front edge to drag on the bottom of the bore, creating a backward force below the center of gravity. Since the projectile is already at an angle the opposing accelerating and drag forces create enough torque on the projectile to cause it to flip over.

If foam mounting tape is used to hold the angled magnets, the strip of wood in the bore may need to be thick the compensate for the foam's thickness.

Rare earth magnets can be used in place of ceramic magnets, but they are so strong that mounting them becomes very difficult. They are also considerably more expensive.

The magnetic projectile is a stack of four rare earth cube magnets, again, available from Amazon for $6. They are 1/2 inch on each edge.

Rare earth magnets have to be treated with respect because there are two very real hazards associated with them. First, they are so strong that they can cause painful pinching if skin gets trapped between them. This can be strong enough to cause bruising and even break skin. Second, The shiny metal plating flakes off very easily, These flakes, as well as the edges left on the magnets, are razor sharp and can easily slice fingers.

Four cube magnets are used because all of the 2-inch long solid rare earth magnets available are magnetized through their sides and because of this won't work.

The large black ceramic magnets are placed so that both angle stacks have the same poles facing the breach. In the image below the north pole in both stacks is pointing to the right

The projectile is gently pushed into the gun's breach with its north pole facing to the left until the magnets grab it and throw it out the opposite end on the left. How they work is easily understood by looking at the magnetic field lines. Because like magnetic poles repel, at first the north poles of the base magnets push against the north pole of the projectile. Once the front face of the projectile is pushed past most of the first pair of magnets, it feels more pull on the south pole than repulsion on the north pole. At this point the projectile is pulled into the gun. Once inside, its right-pointing magnetic field is repelled by the left-pointing magnetic field in the bore. This causes the projectile to be accelerated to the left. As it leaves the bore, the magnetic fields reverse and the projectile is slowed, but by then it has enough momentum to break through and slide freely to the left.

Using a scale made of rubber bands and a nonmagnetic frame the force on the projectile can be measured as it travels down the bore. The following image shows the direction and strength at various positions of the front fave of the projectile:

Angling the magnets means the bore is closer to the north poles than the south, resulting in the net field pointing to the left. This same effect also results in a stronger initial push to the left than the final slowing push to the right at the end of the bore.

If the magnets are aligned perpendicular the the guide strips, the projectile will be pulled to their center and trapped. The gun will work if the magnets are angled 30 degrees from the perpendicular but the range isn't as great. Most magnetic guns use an angle of 45 degrees. At 60 degrees from the perpendicular the projectile travels 80-percent of the distance it does when the magnets are set at 45 degrees. This suggests the ideal angle may be something close to 50 degrees.

How far the projectile travels is determined by the number of angled magnet pairs. The following list provides some idea of how far the projectile travels from the end of the last magnet:

1 magnet pair n= 0 distance (the projectile is captured by the magnet)

2 magnet pairs = 7 inches

3 magnet pairs = 10 inches

4 magnet pairs = 13 inches

5 magnet pairs = 14 inches

6 magnet pairs = 15 inches

7 magnet pairs = 16 inches

8 magnet pairs = 17 inches

10 magnet pairs = 19 inches

15 magnet pairs = 21 inches

 
These distances were for unlubricated wood. A liberal coating of graphite in the bore and on the board on which the projectile slides can double and even triple these distances.

It is very common for the distances the projectile travels to start decreasing after seven magnet pairs. This is usually caused by the magnet pairs not being perfectly aligned with each other. If one stack is longer than the other, an imbalanced magnetic field results, causing the projectile to lose speed. If the misalignment is more than 1/4 inch the projectile can be captured and come to a complete stop near the end of the bore. As the pairs are being stacked, if one side progressively gets longer than the other it's because the angle block on that side is at a larger angle from the perpendicular than the other. The fix is to shim the first magnet on that side until both stacks are the same length. A shim 0.05 inches thick at the end of the first magnet will reduce the length of a stack of 10 magnets by approximately 3/32-inch.

Even when the magnets are perfectly aligned there may be problems. Some guns work the first time they are assembled. Then later if remade with the same magnets they fail. I believe this is caused by manufacturing variances in the strengths of the magnets, which create nonuniform fields in the gun's bore. The only solution is to play with the ordering of the magnets until the gun fires correctly.

The projectile can be made to travel much further if it is reversed and pushed through the bore. As it nears the exit the projectile will fly to the left and travel over three feet, even more if the surfaces have been graphited. It's like squeezing a watermelon seed between two fingers, it suddenly gets spitted out of the end of the gun. But emotionally the effect is less satisfying. The device loses its "gun" feel. It's about the same as pushing two repelling magnets together then letting one go.

While permanent magnet guns will never be a terror weapon, they are nonetheless unusual enough to be worthy of investing the small amount of time it takes to build one. While this page covers most of the issues related to their performance, there are still several areas open to investigation. Is the optimal angle really 50 degrees? Does lining the bore with Teflon result in greater range than graphite lubrication? How does reducing the thickness of the bore guide rails affect performance? For these reasons permanent magnet guns make excellent science projects for students.

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