are in common use, particularly because the design is used for most of the speakers
used on stereo
The concept behind how a dynamic mike works is rather straightforward:
The microphone/speaker consists of a diaphragm attached to a hollow tube. A magnet is inserted into the tube and is mounted to a stationary base. The hollow tube is free to slide over the magnet, but most of the time an additional flexible membrane is used to seal the back of the tube to the base to keep the diaphragm from shooting off during Wagner's "Flight of the Valkyries" and killing the listener or shooting down the throat of someone hitting a high 'C'. It also keeps dust out of the tube.
A \ WIRE <--- INSERT INTO TUBE
P \ COIL
H |------------------+ ___________________||
R | /////// | | PERMANENT MAGNET || BASE
A | /////// | |------------------||
G |---|----|---------+ ||
M / | | TUBE
/ | |
/ | |
/ +| |-
The microphone works because sound will hit against the diaphragm, which will cause it to vibrate. This vibration will cause the hollow tube to move back and forth in relation to the soundwave hitting the diaphragm. Around the tube is a coil of very thin wire, which has both ends mounted outside of the microphone to make connections to electronics. Most of the time the connection is either off of the base or the diaphragm. Inside the tube is a stationary magnet. Because the coil is vibrating around the magnet, the wires will pick up some electric charge due to passing through a magnetic field (also known as Faraday's Law). This weak signal can be amplified and sent out speakers or recorded. Because it self-generates an electric signal, the dynamic microphone/speaker does not require a separate power source (although any amplifiers typically will).
Dynamic mikes are very cheap to produce, and are found in many low-end applications. Like the carbon microphone, this makes an excellent science fair project for kids.