534 RHEOSTATS AND OTHER BALLAST [H. XIII 



case there are two coils (fig. 291). If an alternating current supply 

 is connected with the primary coil an alternating current can be 

 drawn from the secondary coil. 



The voltage and amperage 

 which can be drawn from the 

 secondary coil will depend upon 

 the electric supply and upon the 

 relative number of turns of wire 

 FIG. 290. USE OF A SPECIAL TRANS- in the primary and in the second- 

 FORMER WITH AN ARC LAMP. ary co ii s . jf t he number of turns 

 G Dynamo. is the same in both, then the 



? anSe^ UrrentCirCUit ' voltage and amperage remain 

 A Arc lamp. practically the same as if the 



The primary of the transformer is -i T , , 



connected to the dynamo while the colls were not Present. In other 

 secondary is connected to the arc words the circuit is in every way 



la The transformer has sufficient "re- almost as if the wire were contin - 

 actance" to serve as a ballast for the uous. If the transformer were 



tO "* " * SteP " Wn Perfect the voltage and amperage 

 would be exactly the same as if it 



were not present. In practice they are a little less, but a good 

 transformer gives an efficiency of 95% to 98%. 



If the secondary coil has a different number of turns from the 

 primary coil then the voltage will vary directly as the ratio of the 

 number of turns in the two coils, and the amperage will vary 

 inversely as that ratio. That is, assuming that there is no loss in 

 the transformer, the watts delivered will remain constant as the 

 product of volts x amperes remains the same. 



For example, suppose the secondary coil has %'th as many turns 

 as the primary coil, then the number of volts across the secondary 

 will be %ih the number across the primary and the number of 

 amperes delivered by the secondary will be four times the number 

 drawn by the primary. If now the primary is connected to a 220 

 volt line there will be a potential difference of one-fourth that 

 number or 55 volts across the terminals of the secondary coil. 

 Suppose the secondary coil supplies 60 amperes, as might be the 

 case with an arc lamp, then the primary coil would draw one-fourth 



