220 ELEMENTS OF ELECTRICAL ENGINEERING. 



and B, Fig. 1 84, represent the fine wire coil and the coarse wire 

 coil respectively of an ordinary I o to I transformer. The fine wire 

 coil is connected across 100 volt supply mains, and of course an 

 electromotive force of 10 volts is induced in the coarse wire coil. 

 One terminal of the receiving circuit is connected directly to one 

 of the supply mains as shown, and the other terminal b is con- 

 nected through the coil BB to the supply main a. This connec- 

 tion may be made so that the 10 volts in BB is added to the 

 supply voltage thus giving no volts across be, or it may be 

 made so that the 10 volts in B is subtracted from the supply 

 voltage thus giving 90 volts across be. In the first case we have 



Supply main 



Supply main 





Fig. 185. 



auto-step-up transformation, and in the other case we have auto- 

 step-down transformation. In the first case the coil A acts as a 

 primary, receiving power from the supply mains ; and the coil 

 BB acts as a secondary, delivering power to the receiving circuit. 

 In the second case the coil BB acts as a primary, receiving 

 power from the current which is forced through it in opposition 

 to its voltage ; and the coil A acts as a secondary, delivering 

 power back to the supply mains. In both cases the power which 

 is actually transformed from A to B or from B to A is equal to 

 10 volts multiplied by the current / which is delivered to the re- 

 ceiving circuit, whereas the total amount of power delivered to 

 the receiving circuit is 1 10 X / in the first case, and 90 x /, in 

 the second case. Therefore a 10 to I transformer rated as a i- 

 kilowatt transformer would suffice for the delivery of 1 1 kilowatts 



