page_364

< previous page | page_364 | next page >

Page 364

| measurement. The second, the instrument portion, indicates the value of the resistance being measured. A simplified circuit diagram of this instrument (described more fully in Chap. 7) is shown in Fig. 17-28C. |

| Wheatstone Bridge |

| A type of circuit used widely for precision measurements of resistance is the Wheatstone bridge (Fig. 17-29A). Three of its elementsR1, R2, and R3are precision variable resistors, and the fourth, Rx, is the resistance whose value is to be determined. The bridge is balanced by varying the resistors so that when the galvanometer switch is closed, there will be no deflection of its needle. As a result, there is no difference of potential or voltage between points b and d. This means that the voltage drop across R1, between a and b, is the same as the voltage drop across R3, between a and d. Similarly, the voltage drops across R2 and Rx are equal. By extension of Ohm's Law, it will be found that the ratio of the resistances in both arms of the bridge are equal, that is |

| Figure 17-29 Wheatstone bridge circuits: (A) Resistance bridge, (B) capacitance bridge, and (C) inductance bridge. |

< previous page | page_364 | next page >