As the coil moves, the vane moves within the airtight chamber. The action of the vane against the air
in the chamber opposes the coil movement and damps the oscillations.
Q13. How can a dArsonval meter movement be adapted for use as an ac meter?
Q14. What is damping?
Q15. What are two methods used to damp a meter movement?
Q16. What value does a meter movement react to (actually measure) when measuring ac?
Q17. What value is indicated on the scale of an ac meter?
An additional advantage of damping a meter movement is that the damping systems will act to slow
down the coil and help keep the pointer from overshooting its rest position when the current through the
meter is removed.
INDICATING ALTERNATING CURRENT
Another problem encountered in measuring ac is that the meter movement reacts to the average value
of the ac. The value used when working with ac is the effective value (rms value). Therefore, a different
scale is used on an ac meter. The scale is marked with the effective value, even though it is the average
value to which the meter is reacting. That is why an ac meter will give an incorrect reading if used to
OTHER METER MOVEMENTS
The dArsonval meter movement (permanent-magnet moving-coil) is only one type of meter
movement. Other types of meter movements can be used for either ac or dc measurement without the use
of a rectifier.
When galvanometers were mentioned earlier in this topic, it was stated that they could be either
electromagnetic or electrodynamic. Electrodynamic meter movements will be discussed at this point.
ELECTRODYNAMIC METER MOVEMENT
An electrodynamic movement uses the same basic operating principle as the basic moving-coil meter
movement, except that the permanent magnet is replaced by fixed coils (fig. 1-15). A moving coil, to
which the meter pointer is attached, is suspended between two field coils and connected in series with
these coils. The three coils (two field coils and the moving coil) are connected in series across the meter
terminals so that the same current flows through each.