The Ohmmeter

Chapter 5.13 The Ohmmeter

Fundamental Electrical and Electronic Principles Third Edition Book
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Fundamental Electrical and Electronic Principles Third Edition Book

  • 170Fundamental Electrical and Electronic Principles(b) error indicated valuetrue valuetrue value 100% so, error for V 1 measurement 111...69754754 100% hence, error 3.65% Ans error for V 2 measurement 7 928 2468 246... 100% and, error 3.95% Ans Note: Although the two error fi gures are fairly close to each other in value, the percentage error in the second measurement is about 10% greater than that for the fi rst. This illustrates the fact that the higher the meter internal resistance, compared with the resistance across which it is placed, the smaller the loading effect, and hence higher accuracy is obtained. Since the internal resistance of DVMs is usually in the order of megohms, in many cases they are used in preference to the traditional moving coil instrument. 5.13 The Ohmmeter As the name implies, this instrument is used for the measurement of resistance. This feature is normally included in multimeters. In the case of the AVO, the moving coil movement is also utilised for this purpose. The theory is based simply on Ohm ’ s law. If a known emf is appliedto a resistor, the resulting current fl ow is inversely proportional tothe resistance value. The basic arrangement is shown in Fig. 5.33 .The battery is incorporated into the instrument, as is the variable resistance, R. metermovementRE Fig. 5.33 To use the instrument in this mode, the terminals are fi rst ‘ shorted ’ together, using the instrument leads. The resistor R is then adjusted so that the pointer indicates zero on the ohms scale. Note that the zero position on this scale is at the righthand extreme of the scale. Having