page_334

< previous page | page_334 | next page >

Page 334

| 17 Instruments |

| Function |

| Instruments are needed to determine whether electrical circuits and equipment are operating safely and efficiently. By measuring the values of the electrical quantities involved, operators can pinpoint what is occurring within circuits and equipment. Comparison of these values with predetermined design specifications and with previously experienced values, allows abnormal or troublesome conditions to be detected before they occur. The data thus obtained not only improves the effectiveness of circuits and equipment but also may be used to develop adequate designs for future demands and requirements. |

| Principle of Operation |

| Modern instruments for the detection and measurement of current and voltage are all based on the interaction between two or more magnetic fields. These fields may be created by a conductor or conductors carrying a current or by a permanent magnet (Fig. 17-1). By combining these basic current and voltage elements, instruments are able to determine values of electrical resistance, power, energy, power factor, and other desired quantities. |

| In their simplest form, many of these instruments are small but accurate motors. In the measurement of most quantities, the tendency for rotation of the small motor is pitted against a spring. In measuring other quantities, the motor is allowed to turn, and the revolutions are counted on a register. |

< previous page | page_334 | next page >