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770 ⏐⏐⏐ METHODS OF ANALYSIS AND SELECTED TOPICS (ac)<br />

E<br />

+<br />

–<br />

C 3<br />

R 1<br />

Galvanometer<br />

R 3<br />

R 4<br />

FIG. 17.45<br />

Capacitance comparison bridge.<br />

R 2<br />

C 4<br />

The bridge of Fig. 17.40 is referred to as a Maxwell bridge when Z5 is replaced by a sensitive galvanometer. This setup is used for inductance<br />

measurements when the resistance of the coil is large enough not<br />

to require a Hay bridge.<br />

Application of Eq. (17.6) in the form:<br />

Z2Z3 � Z4Z1 and substituting<br />

Z 1 � R 1 �0° � X C1 ��90° �<br />

R1XC1 ��90° �j R1XC1 � �� � ��<br />

R1 � jXC1 R1 �j XC1 Z 2 � R 2<br />

Z 3 � R 3<br />

and Z 4 � R 4 � j X L4<br />

we have (R2)(R3) � (R4 � j XL4 ) �� � �<br />

R 2R 3 �<br />

�j R 1R 4X C1 � R 1X C1 X L4<br />

���<br />

R 1�j X C1<br />

or (R 2R 3)(R 1 � j X C1 ) � R 1X C1 X L4 � j R 1R 4X C1<br />

and R 1R 2R 3� j R 2R 3X C1 � R 1X C1 X L4 � j R 1R 4X C1<br />

so that for balance<br />

R1R2R3 � R1XC1XL4 1<br />

�<br />

2pfC1<br />

�j R 1X C1<br />

R1 �j X C1<br />

R 2R 3 � � � (2 pfL 4 )<br />

and L4 � C1R2R3 (17.9)<br />

and R 2R 3X C1 � R 1R 4X C1<br />

so that R4 � � (17.10)<br />

R2R3<br />

�<br />

R<br />

Note the absence of frequency in Eqs. (17.9) and (17.10).<br />

One remaining popular bridge is the capacitance comparison<br />

bridge of Fig. 17.45. An unknown capacitance and its associated resistance<br />

can be determined using this bridge. Application of Eq. (17.6)<br />

will yield the following results:<br />

C4 � C3� R1<br />

�<br />

R<br />

R4 � � R2R3<br />

�<br />

R<br />

(17.11)<br />

(17.12)<br />

The derivation of these equations will appear as a problem at the end of<br />

the chapter.<br />

1<br />

(R 1 �0°)(X C1 ��90°)<br />

���<br />

R 1 � jX C1<br />

1<br />

2<br />

N A

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