3-5
concentrate the flux in the narrow space between the iron core and the pole piece. Current flows into one
hairspring, through the coil, and out the other hairspring. The restoring forces of the spiral springs return
the pointer to the normal zero position when the current through the coil is interrupted. Conductors
connect the hairsprings with the outside terminals of the meter. If the instrument is not DAMPED to
absorb the energy of the moving element, the pointer will oscillate (vibrate) for a period of time before
coming to a stop in its final position. Damping is an energy-absorbing system that prevents this.
Figure 3-3.—Detailed view of the basic D'Arsonval meter movement.
DAMPING.—This is accomplished in many D'Arsonval movements by means of the motion of the
aluminum bobbin on which the coil is wound. As the bobbin rotates in the magnetic field, an
electromotive force is induced into it as it cuts through the lines of force. Induced currents flow in the
bobbin in a direction opposite to the motion; this causes the bobbin to go beyond its final position only
once before stopping. The overall sensitivity of the meter can be increased by the use of a lightweight
rotating assembly (bobbin, coil, and pointer) and by the use of jewel bearings, as shown in figure 3-3.
