Oh sorry to answer your question , the arm. is most times made of steel with cast Al ends. The Al makes it lighter saving friction thus less energy used to spin the shaft. That employs the ER ratings, but that is not of concern here!
The steel reacts to the mag. created by to current through the windings in the Stat. and therefore making the motor spin.
Motors with wound Arm. work the same way just different. LOL

Sorry to be long but I like to know how things work before I destroy them!