INDUCTIVE LOADING FOR TELEPHONE FACILITIES 457 



smaller amount of development effort required, the large economies inherent 

 in the use of the improved core-material could begin to be realized at a much 

 earUer date. This was important because of the rapidly increasing demand 

 for loading during the late 1920's. Secondly, by starting quantity production 

 of the core material for use in the exchange area coils, the factory built up 

 experience in the control of the many complicated new processes that were 

 essential to the performance results which were particularly desirable in the 

 toll cable coils. Also, knowing what could be expected from the commercial 

 production of the core-material, the design engineers were in a better posi- 

 tion to specify the most advantageous core-proportions in the final toll 

 cable designs. 



The large demand for M88 loading, relative to that for the heavier weights 

 of exchange area loading, resulted in the concentration of the early develop- 

 ment work on smaller 88 mh loading coils. 



A comparison of the electrical and dimensional characteristics of the new^ 

 permalloy-core coils is given in Table XI (page 460), following the general 

 description of the new designs. 



19.2 612 Coil for M88 Loading 



The new permalloy-core 612 coil became available for a trial installation 

 late in 1926 and quantity production built up to a new high level for ex- 

 change area coils during 1927. 



The size reduction made possible by the favorable permalloy character- 

 istics of high permeability in combination with low losses was carried to a 

 greater degree in the 612 coil than was feasible in the toll cable designs. It 

 was somewhat less than one-fourth as large as the 602 coil in volume and 

 weight. Coil G in the headpiece is a 612 coil. 



The careful cost-equihbrium study that was made to determine the com- 

 mercial design requirements resulted in the 612 coil having a sHghtly smaller 

 d-c resistance than the 602 coil. The resistance-frequency characteristics 

 were sufficiently close to those of the 602 coil to warrant the acceptance of 

 the new coil as an "equivalent" design, with respect to plant engineering. 



The development of the 612 coil involved new design and manufacturing 

 problems beyond those encountered in the design and manufacture of the 

 improved core-material. To make feasible the small size of the toroidal 

 core, an entirely new type of winding machine suitable for high-speed wind- 

 ing to an inner diameter of about 0.75 inch had to be made available. The 

 use of small cores also made it desirable to have a better space-factor in 

 the copper winding. This was achieved by using a composite (conductor) 

 insulation of black enamel and single cotton, instead of the double serving 

 of cotton employed in previous, much larger, designs. Subsequently this 

 change was incorporated in the designs of all small loading coils. 



