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Page 335

Figure 17-1
Operating principle of permanent-magnet,
moving-coil instrument: (A) No current in coil
and hence no torque, and (B) direct current in coil
with a clockwise torque developed by interaction
of coil flux with magnetic flux (broken lines and
arrows show direction and distribution of flux)
(Courtesy, General Electric Co.)

| Developments in solid-state technology may make practical corresponding instrumentation based on electronics rather than on electromagnetics. Present type electromagnetic-type instruments, however, will continue to be employed in industry for a long time to come. |

| Ammeters |

| Permanent Magnet-Coil Instrument.. |

| An ammeter makes use of the interaction between the fields of a permanent magnet and a current-carrying coil (Fig. 17-2). The interaction tends to produce a rotation in the coil while springs on either side resist the rotating force. The amount the pointer of the ammeter moves depends on where the two forces balance; its final position is a measure of the current flowing. This |

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