DAMPING is used to smooth out the vibration and to help prevent overshooting of the meter
pointer. Damping in a dArsonval meter movement is accomplished by the emf caused by the coil
movement. A second damping system uses a vane attached to the coil in an airtight chamber. A meter
movement reacts to the average value of ac, but the scale is calibrated to read the effective (rms) value.
There are meter movements that will measure either ac or dc without the use of a rectifier. They are
the ELECTRODYNAMIC, MOVING-VANE, and HOT-WIRE or THERMOCOUPLE movements.
Electrodynamic meter movements are usually used in wattmeters. They operate much like a
dArsonval meter movement, except field coils are used instead of a permanent magnet.