146 BELL SYSTEM TECHNICAL JOURNAL 



cuits are used almost exclusively in the telephone plant. For sim- 

 plicity, some of the terminal apparatus connected with these circuits 

 is constructed unbalanced, so that it is necessary to interpose trans- 

 formers between the lines and the office equipment, these transformers 

 providing a barrier to the propagation of the relatively large longi- 

 tudinal currents from the line circuits. (Longitudinal currents, in 

 contrast with the usual circulating currents, are currents that flow 

 equally and in the same direction in both sides of the line.) In order 

 to insure that the voltage impressed on the office equipment is attrib- 

 utable to the voltage between the wires of the line circuit and not to 

 that between the wires and ground, it is necessary that the transformers 

 be balanced very carefully; and for certain types of circuits, shields 

 must be interposed so that the direct capacitance between the line 

 Avinding and office winding is reduced to a very small value. 



With a greater emphasis on carrier frequency transmission, a higher 

 degree of balance is required between certain transformer windings, 

 and highly effective shielding is frequently necessary. It is necessary 

 also that the line windings be balanced very closely with respect to 

 capacitances to the shield and case. The unbalance effects in carrier 

 transformers now have been reduced to values in the order of 1 or 2 

 microamperes in circulating current per volt between the line windings 

 and ground at 30,000 cycles per second, which compares with values 

 of 50 microamperes or more for older transformers. At the same time 

 the electrostatic shielding between the windings has been improved to 

 such an extent that the direct capacitance between windings has been 

 reduced to 1 or 2 micromicrofarads instead of 30 or 40 as before. The 

 shields are arranged to intercept the dielectric flux lines tending to con- 

 nect the primary and secondary windings, so that the direct capacitance 

 between the two windings is attributable only to stray flux which by- 

 passes the shield. One of the windings usually is enclosed completely 

 in lead or copper foil with overlapping edges insulated to prevent a 

 short-circuited turn. Still further improvements are obtained by cov- 

 ering the leads with grounded metal braiding, and in special cases by 

 enclosing the terminals of the shielded winding in a separate shielded 

 compartment. In certain transformers designed for high precision 

 testing equipment, the direct capacitance between windings has been 

 reduced to values less than 0.001 micromicrofarad. 



In connection with phantom circuits, severer crosstalk requirements 

 have necessitated more precise balances in the associated voice fre- 

 quency transformers. In these transformers the turns are so arranged 

 that the various distributed capacitances, flux linkages, and d.-c. 

 resistances are disposed symmetrically with respect to ground. It has 



