How does this kind of DC motor work?

[jhast1]

Homework Statement

I used a similar motor to this one in a prac, and then realized that the arms either side of the coils were not magnetised. I was wondering where the magnetic field comes from, and without any special equipment is it possible to calculate it?

Homework Equations

t=bain

 

[rcgldr]

Isn't the purpose of the coil at the bottom of the motor to produce a magnetic field?