A SHIELDED BRIDGE FOR INDUCTIVE IMPEDANCE 



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Purpose of Shielding 

 The principal difficulties in attaining a satisfactory degree of pre- 

 cision in inductance measurements at relatively high frequencies by 

 means of unshielded bridges are those due to the presence of residual 

 or stray admittances existing between the bridge parts or from them 

 to ground. All these parts have quite appreciable surface dimensions 



Fig. 3 — Balance element of shielded bridge. Front view of panel and case 



and when exposed at the usual separations to each other or to ground, 

 have corresponding direct and grounded admittances. Leads to the 

 source of testing current and to the balance detector also introduce 

 rather large admittances. In a bridge intended for rapid operation, 

 the parts subject to manipulation must be arranged compactly and 



Fig. 4 — Inductance standard 



conveniently to the operator. This makes it impracticable to isolate 

 them sufficiently to make the admittance values between these parts 

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