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?