# How does a ferromagnetic object entering a solenoid affect the magnetic field?

So I've been wondering about making a coilgun, and I see that one common thing that has to happen in a coilgun is that a coil has to be rapidly switched on and off, before the iron projectile goes too far and is "stopped" by the electromagnetic field.

I'm trying to figure out how I could control the circuit such that the coil will turn off before the projectile reaches or passe the centre of the magnetic field, and I asked my physics teacher, who said that I should look into using a secondary coil.

His suggestion was that if I were to wrap a secondary around the coil that acts as an electromagnet, when the iron projectile passes through the primary coil (Which is wrapped around a non-conductive PVC pipe), it will affect the magnetic field in a detectable way.

I'm not quite sure how one would go about detecting this change -- hell I'm not even sure what this change is.

Could somebody explain it to me in simpler terms?

Basically, what detectable change would there be on the secondary?

And would it be viable to use this as a method to detect when a projectile enters the coil?

## 1 Answer

A second coil probably is a good way to detect the passing of the projectile. If you have two coils wrapped around the barrel like so:

  coil 1    coil 2
==//////|===/////|===
|     |   |    |
+    GND  A    B


A voltage is applied across coil 1 to accelerate the projectile. This acceleration is caused by a change in the magnetic field inside that coil. Let's say it is in the rightward direction. If this is to accelerate the projectile to the right, the projectile's magnetic field must be directed left. As this projectile approaches coil 2 from the left, it causes an increasing, leftward magnetic field in coil 2. This, in turn, induces a voltage in coil 2 that will oppose the changing magnetic field. So there will be a current in coil 2 that creates a rightward magnetic field. Depending on which way you wrap the coil, you will get a positive or negative voltage across coil 2. As the projectile gets farther away, you will see the opposite voltage induced, as the magnetic field of the projectile is still directed to the left, but is decreasing in magnitude. This can be measured with a voltmeter (attach the leads to A and B) just to see the effect. If you want to use that voltage to switch the first coil off, you will need to create a circuit or use a microcontroller such as an Arduino.

Long answer short, in the above configuration, you will see a voltage induced in the secondary coil as the projectile approaches, and the opposite voltage as it gets farther away.

• Okay, let me get this straight. Coil 1 induces a leftward magnetic field in the projectile while coil 1 is on, and when the projectile approaches coil 2, it creates the same field in coil 2. Wouldn't coil 1 also induce this magnetic field? And when you say farther away, you mean farther away from the centre of the magnetic field of coil 1, correct? Also, you mean that coil 2 is beside coil 1, not wrapped around coil 1? Mar 24, 2015 at 1:44
• The projectile accelerates through coil 1 and moves toward coil 2, which creates a voltage across coil 2. The Arduino can detect this voltage and shut off coil 1. Try experimenting with the distance between coils. The problem with putting coil 2 around coil A is that the magnetic field of the coil will overwhelm the magnetic field produced by the projectile. That is why I suggest putting coil 2 farther along the barrel. Mar 24, 2015 at 1:49