BASIC METERS

concentrate the flux in the narrow space between the iron core and the pole piece. Current flows into onehairspring, through the coil, and out the other hairspring. The restoring forces of the spiral springs returnthe pointer to the normal zero position when the current through the coil is interrupted. Conductorsconnect the hairsprings with the outside terminals of the meter. If the instrument is not DAMPED toabsorb the energy of the moving element, the pointer will oscillate (vibrate) for a period of time beforecoming 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 thealuminum bobbin on which the coil is wound. As the bobbin rotates in the magnetic field, anelectromotive force is induced into it as it cuts through the lines of force. Induced currents flow in thebobbin in a direction opposite to the motion; this causes the bobbin to go beyond its final position onlyonce before stopping. The overall sensitivity of the meter can be increased by the use of a lightweightrotating assembly (bobbin, coil, and pointer) and by the use of jewel bearings, as shown in figure 3-3.3-5