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

+<br />

E<br />

–<br />

I 3<br />

I 1<br />

Z 1<br />

Z 3<br />

I = 0<br />

+ –<br />

V = 0<br />

Z 2<br />

Z 4<br />

FIG. 17.44<br />

Investigating the balance criteria for an ac<br />

bridge configuration.<br />

I 2<br />

–<br />

I 4<br />

However, substituting Y 1 � 1/Z 1, Y 2 � 1/Z 2, Y 3 � 1/Z 3, and Y 4 �<br />

1/Z 4, we have<br />

or Z1Z4 � Z3Z2 VZ5 � 0<br />

�<br />

corresponding with Eq. (17.3) obtained earlier.<br />

Let us now investigate the balance criteria in more detail by considering<br />

the network of Fig. 17.44, where it is specified that I and V � 0.<br />

Since I � 0,<br />

N A<br />

(17.5a)<br />

and I2 � I4 (17.5b)<br />

In addition, for V � 0,<br />

1<br />

� Z1Z 4<br />

I 1 � I 3<br />

I 1Z 1 � I 2Z 2<br />

(17.5c)<br />

and I3Z3 � I4Z4 (17.5d)<br />

Substituting the preceding current relations into Eq. (17.5d), we have<br />

I 1Z 3 � I 2Z 4<br />

and I2 � �I1 Z4<br />

Substituting this relationship for I 2 into Eq. (17.5c) yields<br />

I1Z1 � � I1�Z Z3 2<br />

and Z 1Z 4 � Z 2Z 3<br />

as obtained earlier. Rearranging, we have<br />

� Z1<br />

� � �<br />

Z<br />

Z2<br />

�<br />

Z<br />

3<br />

(17.6)<br />

corresponding with Eq. (8.4) for dc resistive networks.<br />

For the network of Fig. 17.42, which is referred to as a Hay bridge<br />

when Z 5 is replaced by a sensitive galvanometer,<br />

Z 1 � R 1 � jX C<br />

Z 2 � R 2<br />

Z 3 � R 3<br />

1<br />

� Z3Z 2<br />

� Z4<br />

Z 4 � R 4 � jX L<br />

This particular network is used for measuring the resistance and inductance<br />

of coils in which the resistance is a small fraction of the reactance<br />

X L.<br />

Z 3<br />

4

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