TRANSFORMERS 195 



frequency. Having selected one of the group as a 

 standard, connect the primaries of all the transformers 

 in series. Short circuit all the secondaries as shown in 

 Fig. 65; then connect the ammeter to the secondaries 

 of the standard and check the transformer. This con- 

 nection must be made with lamp cord or other suitable 

 wire, and must be sufficiently long to allow the ammeter 

 to be at least ten feet from the primary circuit in order to 

 avoid the effects of stray field. Check the reading of 

 the ammeter connected to the standard by means of 

 another ammeter, bring the current up to normal and 

 note the reading of the check ammeter. Now transfer 

 the ammeter on the standard to another transformer 



To current Supp/y 



\ I 



Secondaries 



Ammeters 



Fig. 65 

 Connections for Ratio of Series Transformer 



and bring the current up to the reading previously noted 

 or the check ammeter. When correct, read the ammeter 

 on the transformer in test ; if this reading agrees with the 

 reading when the ammeter was on the standard the ratio 

 is correct. Proceed in this manner until all the trans- 

 formers have been checked with the standard. 



New check readings are often necessary, due to unequal 

 heating of meters and lines. Ratios should check within 

 one per cent. Keep all secondaries short circuited except 

 those to which the meters are connected. A few trans- 

 formers of this type are built with two sets of windings ; 

 these should be tested as though they were two separate 

 transformers. 



