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BELL SYSTEM TECHNICAL JOURNAL 



that a high ampHfication, in this instance 10,000 to 1 in impedance 

 level from a 30-ohm source, may be realized without undue restriction 

 in either the low- or the high-frequency transmission. 



Since transformers of this type are located at points of very low 

 energy levels, special pains must be taken to avoid interference from 

 stray magnetic and electrostatic fields. To prevent hum from nearby 

 power apparatus, transformers are enclosed in cases or shields of high 

 permeability material. The interference voltages induced in these 

 transformers are some 30 or 40 decibels less than in older unshielded 

 types of transformers. For higher frequency interference, effective 

 shielding is obtained by cases made of high-conductivity material such 

 as copper or aluminum. 



Reduction in Size and Weight 



The demand for lightweight equipment for aircraft communication 

 and for portable apparatus for testing and recording has resulted in 

 the development of communication transformers of unusually small 

 size and weight. The smaller sizes weigh only 3| ounces and occupy 

 a space of but 3 cubic inches. One of these used in aircraft receiving 

 sets is illustrated in Fig. 5 contrasted with an earlier transformer also 

 for lightweight service. The transmission loss-frequency character- 

 istics are shown in Fig. 6. The corresponding characteristic of an 

 input transformer for similar service is shown in Fig. 7. 



Fig. 5 — Output transformer A (right) utilizing a permalloy core transmits fre- 

 quencies from 40 to 3,000 cycles with greater over-all efficiency than the larger output 

 transformer B (left) utilizing a core of silicon steel (see Fig. 6). 



