A SHIELDED BRIDGE FOR INDUCTIVE IMPEDANCE 159 



meet these requirements, the bridge coils were constructed as follows. 

 The spool used is a glass cylinder ^^ in. in diameter. The winding is 

 applied as follows: Starting at one end of the spool, a single strand of 

 the wire is wound on until 14 inductive turns have been applied giving 

 a resistance of approximately 50 ohms. Then the wire is tied, the 

 direction of winding reversed, and an exactly equal number of turns 

 wound over the first 14, but in an opposite direction. This brings the 

 wire to the beginning. It is again tied, carried parallel to the axis 

 of the spool over this first section and a second section wound. This 

 is continued until ten sections have been applied. A thin sheet of 

 mica is tied in place around the winding and the projecting ends of the 

 wire bared of insulation. The whole is then baked to anneal the wire 

 and dry the insulation. While hot, it is dipped several times in molten 

 asphalt compound until a continuous coating of this moisture-proof 

 material has been formed over the winding and surrounding mica 

 wrapping. Adjustment for resistance balance is made by varying the 

 length of the two wire ends. 



The effective reactances of coils made as above are positive before 

 assembly in their shields. The effect of the shield is to increase the 

 capacitance. Table II gives data obtained on the two coils made for 



Fig. 12 — Ratio arms 



the bridge and shows the uniformity of phase-angle difference main- 

 tained by these coils over the operating frequency range. Final 

 adjustment for reactance balance is made with the coils in the bridge 

 circuit, a small amount of inductive coiling of the terminal leads 

 sufficing for this purpose. In establishing this balance, use is made 

 of the reversing switch described in the following section. Fig. 12 

 shows these coils assembled in their shields. 



