3-2METER POWER SOURCEMeters are either SELF-EXCITED or EXTERNALLY EXCITED. Self-excited meters operate fromtheir own power sources. Externally excited meters get their power from the circuit to which they areconnected. Most common meters (voltmeters, ammeters, and ohmmeters) that you use in your workoperate on the electromagnetic principle. All measuring instruments must have some form of indicatingdevice, usually a meter, to be of any use to you. The most basic indicating device used in instruments thatmeasure current and voltage operates by using the interaction between the magnetic fields associated withcurrent flow in the circuit. Before continuing, you might want to review the properties of magnetism andelectromagnetism in NEETS, Module 1, Introduction to Matter, Energy, and Direct Current.Q-1.What meters operate from their own power sources?BASIC METER MOVEMENTA stationary, permanent-magnet, moving-coil meter is the basic meter movement used in mostmeasuring instruments used for servicing electrical equipment. When current flows through the coil, aresulting magnetic field reacts with the magnetic field of the permanent magnet and causes the movablecoil to rotate. The greater the intensity of current flow through the coil, the stronger the magnetic fieldproduced; the stronger the magnetic field produced, the greater the rotation of the coil. TheGALVANOMETER is an example of one type of stationary, permanent-magnet, moving-coil measuringinstrument.GalvanometerThe galvanometer is used to measure very low currents, such as those in bridge circuits. In modifiedform, the galvanometer has the highest sensitivity of any of the various types of meters in use today. Asimplified diagram of a galvanometer is shown in figure 3-1. It is different from other instruments usedfor the same purpose because its movable coil is suspended by means of metal ribbons instead of a shaftand jewel-bearing arrangement often used in other instruments.Figure 3-1.—Simplified galvanometer.
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