School me on Field coil speakers....

[1blueheron]

Have been seeing a lot of mentions and schematics that show field coil speakers.  How are these identified and do they have any virtues or applications that make them better than a modern day speaker?

[sluckey]

They are identified by a big coil of wire (electromagnet) on the back of the speaker rather than a chunk of magnetic metal (permanent magnet). They have no qualities worth preserving unless you are restoring some antique and want to keep it as original as possible. Google may have a different opinion.   

[byoungblood]

Having dealt with a couple in some old radios, they're a bit of a pain to work with. Their field also doubles as a choke for the power supply in every one I've seen, so if you run across something that has one, you've got to compensate for the DC resistance of the field coil should you replace it with a permanent magnet speaker.