A' B B'C C' D D' 



S -A] STUDY AND OPERATION. J 35 



these only. In the auto-transformer,* or single-coil transformer, 

 part of the windings is common to both primary and secondary. 

 Connect the transformer coils as an auto-transformer, and 

 verify the different values of voltage transformation. To do 

 this connect all coils in series and consider any one or more of 

 the coils, as may be desired, to be 



110 Volt Supply 



primary or secondary. Some of 



the coils will at the same time 



form part of both primary and 



secondary; these coils will carry, 



therefore, both the primary and LAAAA 



the secondary currents, which are Load 



opposite in phase ( zoa) and so FlG<2 . Step _ down 



give a resultant current approxi- former, using coils A B C D as 



mately equal to the arithmetical P*T; coil D is also used as 



secondary. 



difference of the two. 



12. Connect the coils as a step-down auto-transformer (Fig. 

 2) and as a step-up auto-transformer or "booster" (Fig. 3). 

 Using suitable resistances as a load, determine the current! in 

 each coil, in the resistance and in the supply line and explain 

 their relative values. The currents and voltages for other com- 

 binations of coils can be computed and compared, or determined 

 experimentally. Suppose a 3 : 2 ratio is desired ; with A, B, C 

 as primary, how would the use of C, D as secondary compare 

 with the use of B, C? 



13. Advantages of the Auto-transformer. It will be found 

 that the auto-transformer requires less copper than a transformer 

 with separate primary and secondary coils; it has, therefore, not 

 only lower first cost but less copper loss and copper drop, giving 

 better efficiency and regulation. The saving in space on account 



* Also called " balance coil " or " compensator " ; the term auto-converter 

 should be discarded. 



t In making measurement of current, it will be found convenient to use 

 one ammeter and a 3-way ammeter switch. 



