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BELL SYSTEM TECHNICAL JOURNAL 



TABLE II 

 Effective Inductance of Ratio Arm Coils 



Resistance of each coil = 1051.2 ohms 



Reversing Switches. As will be noted from the diagram showing 

 the circuit arrangement of the reversing switches, these are required 

 to be completely enclosed in a shield which is connected to the junction 

 point D of the bridge. They must also, of course, be subject to 

 manipulation. 



Obviously, then, this shield must be supported in some fashion 

 from the outer enclosing ground shield of the bridge. The admittance 

 between these two shields is a direct shunt on either one half or all of 

 the impedance arm CD. While the capacitance component of this 

 admittance can be readily balanced, it is more difficult to balance the 

 conductance component, since the latter varies irregularly with fre- 

 quency. Consequently, it is desired to make this factor so low that 

 it can ordinarily be neglected. The construction adopted for this 

 purpose is shown in the illustration. Fig. 13, which is a partially 

 assembled view of the two reversing switches, their shield and its 

 supporting brackets. It will be noted that the shield is supported by 

 the brackets by means of small glass rods (four of which are shown). 

 The low phase-angle characteristics of glass make it a favorable ma- 

 terial to use, from this standpoint, but from the standpoint of machin- 

 ing into shape suitable for insulating supports, it is not so good. The 

 construction shown, however, adapts it very well to this purpose. 

 One other feature is worthy of special note; that is, the small change 

 in position of any of the switch parts which occurs in effecting a rever- 

 sal, the only metallic part moving being the small metal segments of 

 the rotating disk. 



Transformers. The transformers used for isolating the bridge circuit 

 electrostatically from the source of testing current and from the 

 detector system should have substantially zero external electro- 

 magnetic fields. This is to prevent inductive coupling to other parts 



