NEW SINGLE CHANNEL CARRIER TELEPHONE SYSTEM 183 



the air core solenoidal coils used in the Type D system. Since these 

 new coils have less dissipation, the filters have flatter and wider trans- 

 mission bands which contribute to improve telephone quality in the 

 system. These coils are toroidal in shape and have very small stray 

 fields and therefore small coupling with any nearby coils. This 

 permits them to be packed closely together, which results in a large 

 decrease in the size of the filters. A further reduction in size and 

 cost of the filters was effected by dispensing with individual containers 

 for each coil. The elements of the filter are wired together and held in 

 place in the filter can by a potting compound. This is poured around 

 them, hermetically sealing the whole assembly. 



The small size of the elements and the very low coupling between 

 them permit the assembly of more than one filter in the same can. 

 For example, by a careful placing of the elements it was possible to 

 place the transmitting and receiving low-pass filters and the modulator 

 output filter in one can approximately 3| inches by 4j inches by 4j 

 inches in size. A photograph of a high and low-pass line filter with the 

 can cut away is shown in Fig. 17. The filters for a terminal of this 

 system require 70 square inches of mounting space or about 1/5 of 

 that required for those of a Type D system. 



Conclusions 



The development of the Type H system is another step in extending 

 the use of carrier systems. Improvements in performance and simplifi- 

 cations which are effective in reducing its cost as compared with the 

 Type D system which it supersedes have been obtained. Reduction 

 in size and provision for operation on a-c. supply simplify its installa- 

 tion, particularly in outlying offices where suitable d-c. power supply 

 is not ordinarily available. Its portability makes it well suited to 

 provide additional circuits required in cases of emergency. The 

 Type H system is expected to have a large application in the Bell 

 System telephone plant, and in addition to provide carrier circuits for 

 the communication systems of other companies. 



