TRANSFORMERS. 



[Exp. 



110 Volt Supply 



Load Resistance 



A B C as primary ; coils A B CD 

 are used as secondary. 



of less copper makes it possible to reduce also the iron and 

 iron loss. 



This advantage of an auto-transformer will be seen to be 

 greater the nearer the ratio of transformation is I : i. For a 

 comparison of output of transformers and auto-transformers, 

 see 8, 9, Exp. -B. An auto-transformer cannot be used 



when complete insulation of the 

 primary from the secondary is 

 necessary, as in house lighting from 

 high potential lines. 



The step-down auto-transformer 

 of Fig. 2 is in common use as a 

 starting device for induction mo- 



FIG. 3 . Step-up auto-trans- tors > g ivin g a lower voltage than 

 former or booster, using coils full line voltage while the motor 



is coming up to speed; see Fig. 6, 

 Exp. 7- A. 



A common use of the step-up arrangement of Fig. 3 is as a 

 booster to raise the voltage on remote parts of a distribution 

 system, say from 2,000 to 2,200 volts. For this a standard 

 2,000/200 volt transformer can be used, with the low-potential 

 coil in series with the primary to boost the voltage, as in Fig.2, 

 Exp. 7~B. This becomes a "negative booster" if the connec- 

 tions of the low-potential coil (coil D in Fig. 3) are reversed. 

 (If a standard transformer is to be tried in the laboratory, a 

 loo-volt circuit may be boosted to no volts, or reduced to 

 90 volts.) 



14. Constant Potential Operation. Transformers are usually 

 operated from a constant potential circuit, so as to transform- 

 either step-up or step-down from a constant primary potential 

 to a constant secondary potential. 



15. Open Circuit. Connect a no-volt alternating current 

 supply circuit across two of the transformer coils in scries, as 



