IMPROVEMENTS IN COMMUNICATION TRANSFORMERS 13 7 



more elaborate equivalent network than the customary 11 or T net- 

 work. With the use of such a network, the performance of the trans- 

 former may be correlated mathematically with its constants in accord- 

 ance with network theory. 



Improvements in Low-Frequency Transmission 



The great improvements in communication transformers, particu- 

 larly at audio frequencies, are largely attributable to the invention 

 and application of the permalloys as magnetic core materials. For 

 convenience, the term "permalloy" has been applied to a group of 

 nickel-iron alloys containing between 30 and 95 per cent nickel which 

 have been developed by Bell Telephone Laboratories.^- ^' ' In addi- 

 tion to other desirable magnetic properties, some of these permalloys * 

 when properly heat treated yield exceptionally high initial permea- 

 bilities. As a result of the use of these special alloys, telephone trans- 

 formers may be designed to have less loss and distortion over wider 

 frequency ranges than has been possible in transformers designed 

 without the benefit of use of such materials. 



Figure 1 illustrates the excellence of performance resulting from the 

 use of a special permalloy consisting of approximately 4 per cent 



100 500 1000 5000 10.000 



FREQUENCY IN CYCLES PER SECOND 



50,000 



Fig. 1 — Transmission-frequency characteristic of a transformer utilizing a perm- 

 alloy core and designed to connect a telephone transmission line to a program re- 

 peater. Transmission loss shown is the loss relative to an ideal transformer of the 

 same ratio. 



chromium, 78 per cent nickel, and the remainder iron, in a transformer 

 designed to connect a telephone transmission line to a program re- 

 peater. Figure 2 shows the voltage amplification characteristic of an 

 interstage transformer for a high quality amplifier. As is well known, ^ 

 superimposed direct currents generally decrease the elTective a.-c. per- 

 meability of ferromagnetic materials. Therefore, to retain the full 

 benefit of the permalloy core, an auxiliary circuit was used with this 

 transformer for supplying the plate current to the preceding tube. 

 Another illustration is a transformer designed for use as an input 

 transformer (that is, one designed to operate into the grid circuit of a 



