page_336

< previous page | page_336 | next page >

Page 336

| Figure 17-2 Basic parts of permanentmagnet, moving-coil instrument (Courtesy, General Electric Co.) |

| type of ammeter, although usually used to measure dc, can also be used to measure ac. |

| Related to the permanent-magnet instrument in principle is the moving-magnet instrument (Fig. 17-3). In this instrument, the coil is stationary, and the magnet is movable. Springs are not necessary here as the moving element is kept from turning by the field of the two permanent control magnets. When current flows through the coil, it sets up a magnetic field that interacts with the magnetic field of the control magnets. The magnetic field of the rotating magnet will align itself with the resultant magnetic field, and the instrument pointer moves up the scale. |

| Electrodynamometer Instrument. |

| In the electrodynamometer, the permanent magnet is replaced with a stationary coil (Fig. 17-4). The strength of its magnetic field, therefore, is not constant but varies with the amount of current in the coil. The tendency for rotation depends not only on the field set up by the current in the rotating coil but also on that set up by the stationary coil. This type of element provides more accuracy in the measurement of low values of current. Curves showing the torque or rotating tendency it develops for current and voltage measurement are shown in Fig. 17-5. Use of an electrodynamometer as an ammeter is shown in Fig. 17-6. |

| Voltmeters |

| A voltmeter is simply an ammeter to which a fixed (and usually large) resistance is connected in series with the coils. Deriving from Ohm's Law, |

< previous page | page_336 | next page >