. The Bell System technical journal . al that it be constant. Theconductance component should be negligibly small. To attain thisend, the shields are separated at definite distances by means of hardrubber rings turned to fit the outer corners of the inner shield and thecorresponding inner corners of the enclosing shield. These rings aremade of the smallest cross-section consistent with mechanical strengthrequirements so as to introduce the minimum amount of solid materialinto the space between the shields. This minimizes the capacitance11 162 BELL SYSTEM TECHNICAL JOURNAL and conductance value
. The Bell System technical journal . al that it be constant. Theconductance component should be negligibly small. To attain thisend, the shields are separated at definite distances by means of hardrubber rings turned to fit the outer corners of the inner shield and thecorresponding inner corners of the enclosing shield. These rings aremade of the smallest cross-section consistent with mechanical strengthrequirements so as to introduce the minimum amount of solid materialinto the space between the shields. This minimizes the capacitance11 162 BELL SYSTEM TECHNICAL JOURNAL and conductance values. These shields must not, of course, be allowedto act as short-circuited secondaries on the transformer which wouldbe the case if they linked conductively with the windings. Each istherefore made in two parts similar to toroidal channels which uponassembly have their overlapping inner circumferences insulated fromeach other by means of thin mica laminations. Further details of theconstruction will be evident from a study of Fig. Fig. 14—Shielded transformer The windings are, of course, proportioned so as to connect with areasonable degree of efficiency the associated impedances. For bestresults two sets of transformers are used to cover the complete fre-quency range, one from 500 to 5000 cycles and the other from 5000to 50,000 cycles. Balancing Condensers arid Impedance Arm Balance. It has beenbrought out previously that two adjustable capacitances are required,one to effect the proper adjustment of the bridge capacitances toground and the other to balance the residual capacitances shunting oneof the impedance arms. Such capacitances are provided in the formof adjustable air condensers each having a maximum value of about500 mmf. The construction used is such as to give a high degree ofstability of capacitance combined with low conductance arrangement of these condensers in relation to the other apparatusis shown in Fig. 3. The effect on the accuracy
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