What is Wattmeter

Chapter 5.14 Wattmeter

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

  • Electromagnetism 171 ‘ zeroed ’ the meter, the ‘ short ’ is removed. Finally, the resistance to be measured is connected between the terminals. The resistance value will then be indicated by the position of the pointer on the ohms scale. It is not recommended that resistance values be measured using this technique! The scale is extremely cramped over the higher resistance range, and is extremely non-linear. This makes accurate measurement of resistance virtually impossible. However, the use of this facility for continuity checking is useful. If you wish to measure a resistance value fairly accurately, then use this facility on a digital multimeter. The same basic principle, of applying a known emf and measuring the resulting current, may still be employed. The internal electronic circuitry then converts this into a display of resistance value. By defi nition, the scale cannot be cramped, and is easy to read. If a resistance has to be measured with a high degree of accuracy, then a Wheatstone Bridge must be used. 5.14 Wattmeter This instrument is used to measure electrical power. In its traditional form it is called a dynamometer wattmeter. It is also available in a purely electronic form. The dynamometer type utilises the motor principle. The meter consists of two sets of coils. One set is fi xed, and is made in two identical parts. This is the current coil, and is made from heavy gauge copper wire. The resistance of this coil is therefore low. The voltage coil is wound from fi ne gauge wire, and therefore has a relatively high resistance. The voltage coil is mounted on a circular former, situated between the two parts of the current coil. The basic arrangement is illustrated in Fig. 5.34 . voltagecoilcurrentcoil Fig. 5.34