These are examples of electrostatic and electrodynamic inductive coupling. Electric violins may use magnetic, piezoelectric, or electrodynamic pickups. An electrodynamic ammeter uses an electromagnet instead of the permanent magnet of the d'Arsonval movement. In the tweeter market, the AMT competes against electrostatic, ribbon and electrodynamic tweeters. The traditional analog wattmeter is an electrodynamic instrument. EMP sources based on the electrodynamic and electromagnetic principles. Their design uses electrodynamic suspension for levitating and eddy current braking. The primary ones used in maglev trains are servo-stabilized electromagnetic suspension (EMS), electrodynamic suspension (EDS). An electrodynamic wheel is a type of wheel proposed for use in electrodynamic levitation maglev. The full apparatus of quantum field theory is often unnecessary for describing electrodynamic systems. However, additional terms arise in an electrodynamic multipole expansion. This heuristic derivation of the electrodynamic level shift following Welton is from "Quantum Optics". The open system electrodynamic tether, which uses the ionosphere, is being researched. Another proposed method is to use spinning electrodynamic tethers instead of hanging tethers. This makes for rather different acoustic behavior in rooms compared to conventional electrodynamic loudspeakers.